/* packet-mbtcp.c
 * Routines for Modbus/TCP and Modbus/UDP dissection
 * By Riaan Swart <rswart@cs.sun.ac.za>
 * Copyright 2001, Institute for Applied Computer Science
 *                   University of Stellenbosch
 *
 * See http://www.modbus.org/ for information on Modbus/TCP.
 *
 * Updated to v1.1b of the Modbus Application Protocol specification
 *   Michael Mann * Copyright 2011
 *
 * Updates Oct/Nov 2012 (Chris Bontje, cbontje[*at*]gmail.com)
 * - Some re-factoring to include support for serial Modbus RTU encapsulated in TCP messages
 * - Minor text/syntax clean-up
 * - Include decoding of holding/input response register data
 * - Optionally decode holding/input registers as UINT16, UINT32, 32-bit Float IEEE/Modicon
 * - Add various register address formatting options as "Raw", "Modicon 5-digit", "Modicon 6-digit"
 *
 * Updates Aug 2013 (Chris Bontje)
 * - Improved dissection support for serial Modbus RTU with detection of query or response messages
 *
 *****************************************************************************************************
 * A brief explanation of the distinction between Modbus/TCP and Modbus RTU over TCP:
 *
 * Consider a Modbus poll message: Unit 01, Scan Holding Register Address 0 for 30 Registers
 *
 * The Modbus/TCP message structure will follow the pattern below:
 * 00 00 00 00 00 06 01 03 00 00 00 1E
 * AA AA BB BB CC CC DD EE FF FF GG GG
 *
 * A = 16-bit Transaction Identifier (typically increments, or is locked at zero)
 * B = 16-bit Protocol Identifier (typically zero)
 * C = 16-bit Length of data payload following (and inclusive of) the length byte
 * D = 8-bit Unit / Slave ID
 * E = 8-bit Modbus Function Code
 * F = 16-bit Reference Number / Register Base Address
 * G = 16-bit Word Count / Number of Registers to scan
 *
 * A identical Modbus RTU (or Modbus RTU over TCP) message will overlay partially with the msg above
 * and contain 16-bit CRC at the end:
 * 00 00 00 00 00 06 01 03 00 00 00 1E -- -- (Modbus/TCP message, repeated from above)
 * -- -- -- -- -- -- 01 03 00 00 00 1E C5 C2 (Modbus RTU over TCP message, includes 16-bit CRC footer)
 * AA AA BB BB CC CC DD EE FF FF GG GG HH HH
 *
 * A = Not present in Modbus RTU message
 * B = Not present in Modbus RTU message
 * C = Not present in Modbus RTU message
 * D = 8-bit Unit / Slave ID
 * E = 8-bit Modbus Function Code
 * F = 16-bit Reference Number / Register Base Address
 * G = 16-bit Word Count / Number of Registers to scan
 * H = 16-bit CRC
 *
 *****************************************************************************************************
 * Wireshark - Network traffic analyzer
 * By Gerald Combs <gerald@wireshark.org>
 * Copyright 1998 Gerald Combs
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#include "config.h"

#include <epan/packet.h>
#include <epan/wmem/wmem.h>
#include "packet-tcp.h"
#include "packet-mbtcp.h"
#include <epan/prefs.h>
#include <epan/expert.h>
#include <epan/crc16-tvb.h> /* For CRC verification */

void proto_register_modbus(void);
void proto_reg_handoff_mbtcp(void);
void proto_reg_handoff_mbrtu(void);

/* Initialize the protocol and registered fields */
static int proto_mbtcp = -1;
static int proto_mbrtu = -1;
static int proto_modbus = -1;
static int hf_mbtcp_transid = -1;
static int hf_mbtcp_protid = -1;
static int hf_mbtcp_len = -1;
static int hf_mbtcp_unitid = -1;
static int hf_mbtcp_functioncode = -1;
static int hf_modbus_reference = -1;
static int hf_modbus_lreference = -1;
static int hf_modbus_reftype = -1;
static int hf_modbus_readref = -1;
static int hf_modbus_writeref = -1;
static int hf_modbus_wordcnt = -1;
static int hf_modbus_readwordcnt = -1;
static int hf_modbus_writewordcnt = -1;
static int hf_modbus_bytecnt = -1;
static int hf_modbus_lbytecnt = -1;
static int hf_modbus_bitcnt = -1;
static int hf_modbus_exceptioncode = -1;
static int hf_modbus_diag_sf = -1;
static int hf_modbus_diag_return_query_data_request = -1;
static int hf_modbus_diag_return_query_data_echo = -1;
static int hf_modbus_diag_restart_communication_option = -1;
static int hf_modbus_diag_return_diag_register = -1;
static int hf_modbus_diag_ascii_input_delimiter = -1;
static int hf_modbus_diag_return_bus_message_count = -1;
static int hf_modbus_diag_return_bus_comm_error_count = -1;
static int hf_modbus_diag_return_bus_exception_error_count = -1;
static int hf_modbus_diag_return_slave_message_count = -1;
static int hf_modbus_diag_return_no_slave_response_count = -1;
static int hf_modbus_diag_return_slave_nak_count = -1;
static int hf_modbus_diag_return_slave_busy_count = -1;
static int hf_modbus_diag_return_bus_char_overrun_count = -1;
static int hf_modbus_status = -1;
static int hf_modbus_event_count = -1;
static int hf_modbus_message_count = -1;
static int hf_modbus_event_recv_comm_err = -1;
static int hf_modbus_event_recv_char_over = -1;
static int hf_modbus_event_recv_lo_mode = -1;
static int hf_modbus_event_recv_broadcast = -1;
static int hf_modbus_event_send_read_ex = -1;
static int hf_modbus_event_send_slave_abort_ex = -1;
static int hf_modbus_event_send_slave_busy_ex = -1;
static int hf_modbus_event_send_slave_nak_ex = -1;
static int hf_modbus_event_send_write_timeout = -1;
static int hf_modbus_event_send_lo_mode = -1;
static int hf_modbus_andmask = -1;
static int hf_modbus_ormask = -1;
static int hf_modbus_data = -1;
static int hf_modbus_mei = -1;
static int hf_modbus_read_device_id = -1;
static int hf_modbus_object_id = -1;
static int hf_modbus_num_objects = -1;
static int hf_modbus_list_object_len = -1;
static int hf_modbus_conformity_level = -1;
static int hf_modbus_more_follows = -1;
static int hf_modbus_next_object_id = -1;
static int hf_modbus_object_str_value = -1;
static int hf_modbus_reg_uint16 = -1;
static int hf_modbus_reg_uint32 = -1;
static int hf_modbus_reg_ieee_float = -1;
static int hf_modbus_reg_modicon_float = -1;
static int hf_mbrtu_unitid = -1;
static int hf_mbrtu_crc16 = -1;

/* Initialize the subtree pointers */
static gint ett_mbtcp = -1;
static gint ett_mbrtu = -1;
static gint ett_modbus_hdr = -1;
static gint ett_group_hdr = -1;
static gint ett_events = -1;
static gint ett_events_recv = -1;
static gint ett_events_send = -1;
static gint ett_device_id_objects = -1;
static gint ett_device_id_object_items = -1;

static expert_field ei_mbrtu_crc16_incorrect = EI_INIT;
static expert_field ei_modbus_data_decode = EI_INIT;

static dissector_handle_t modbus_handle;
static dissector_handle_t mbtcp_handle;
static dissector_handle_t mbrtu_handle;

static dissector_table_t   modbus_data_dissector_table;
static dissector_table_t   modbus_dissector_table;


/* Globals for Modbus/TCP Preferences */
static gboolean mbtcp_desegment = TRUE;
static guint global_mbus_tcp_port = PORT_MBTCP; /* Port 502, by default */
static gint global_mbus_tcp_register_format = MBTCP_PREF_REGISTER_FORMAT_UINT16;
static gint global_mbus_tcp_register_addr_type = MBTCP_PREF_REGISTER_ADDR_RAW;

/* Globals for Modbus RTU over TCP Preferences */
static gboolean mbrtu_desegment = TRUE;
static guint global_mbus_rtu_port = PORT_MBRTU; /* 0, by default        */
static gint global_mbus_rtu_register_format = MBTCP_PREF_REGISTER_FORMAT_UINT16;
static gint global_mbus_rtu_register_addr_type = MBTCP_PREF_REGISTER_ADDR_RAW;
static gboolean mbrtu_crc = FALSE;

static int
classify_mbtcp_packet(packet_info *pinfo)
{
    /* see if nature of packets can be derived from src/dst ports */
    /* if so, return as found */
    /*                        */
    /* XXX Update Oct 2012 - It can be difficult to determine if a packet is a query or response; some way to track  */
    /* the Modbus/TCP transaction ID for each pair of messages would allow for detection based on a new seq. number. */
    /* Otherwise, we can stick with this method; a configurable port option has been added to allow for usage of     */
    /* user ports either than the default of 502.                                                                    */
    if (( pinfo->srcport == global_mbus_tcp_port ) && ( pinfo->destport != global_mbus_tcp_port ))
        return RESPONSE_PACKET;
    if (( pinfo->srcport != global_mbus_tcp_port ) && ( pinfo->destport == global_mbus_tcp_port ))
        return QUERY_PACKET;

    /* else, cannot classify */
    return CANNOT_CLASSIFY;
}

static int
classify_mbrtu_packet(packet_info *pinfo, tvbuff_t *tvb)
{
    /* see if nature of packets can be derived from src/dst ports */
    /* if so, return as found */
    if (( pinfo->srcport == global_mbus_rtu_port ) && ( pinfo->destport != global_mbus_rtu_port ))
        return RESPONSE_PACKET;
    if (( pinfo->srcport != global_mbus_rtu_port ) && ( pinfo->destport == global_mbus_rtu_port ))
        return QUERY_PACKET;

    /* Special case for serial-captured packets that don't have an Ethernet header */
    /* Dig into these a little deeper to try to guess the message type */
    if (!pinfo->srcport) {
        /* If length is 8, this is either a query or very short response */
        if (tvb_length(tvb) == 8) {
            /* Only possible to get a response message of 8 bytes with Discrete or Coils */
            if ((tvb_get_guint8(tvb, 1) == READ_COILS) || (tvb_get_guint8(tvb, 1) == READ_DISCRETE_INPUTS)) {
                /* If this is, in fact, a response then the data byte count will be 3 */
                /* This will correctly identify all messages except for those that are discrete or coil polls */
                /* where the base address range happens to have 0x03 in the upper 16-bit address register     */
                if (tvb_get_guint8(tvb, 2) == 3) {
                    return RESPONSE_PACKET;
                }
                else {
                    return QUERY_PACKET;
                }
            }
            else {
                return QUERY_PACKET;
            }
        }
        else {
            return RESPONSE_PACKET;
        }
    }

    /* else, cannot classify */
    return CANNOT_CLASSIFY;
}

/* Translate function to string, as given on p6 of
 * "Open Modbus/TCP Specification", release 1 by Andy Swales.
 */
static const value_string function_code_vals[] = {
    { READ_COILS,             "Read Coils" },
    { READ_DISCRETE_INPUTS,   "Read Discrete Inputs" },
    { READ_HOLDING_REGS,      "Read Holding Registers" },
    { READ_INPUT_REGS,        "Read Input Registers" },
    { WRITE_SINGLE_COIL,      "Write Single Coil" },
    { WRITE_SINGLE_REG,       "Write Single Register" },
    { READ_EXCEPT_STAT,       "Read Exception Status" },
    { DIAGNOSTICS,            "Diagnostics" },
    { GET_COMM_EVENT_CTRS,    "Get Comm. Event Counters" },
    { GET_COMM_EVENT_LOG,     "Get Comm. Event Log" },
    { WRITE_MULT_COILS,       "Write Multiple Coils" },
    { WRITE_MULT_REGS,        "Write Multiple Registers" },
    { REPORT_SLAVE_ID,        "Report Slave ID" },
    { READ_FILE_RECORD,       "Read File Record" },
    { WRITE_FILE_RECORD,      "Write File Record" },
    { MASK_WRITE_REG,         "Mask Write Register" },
    { READ_WRITE_REG,         "Read Write Register" },
    { READ_FIFO_QUEUE,        "Read FIFO Queue" },
    { ENCAP_INTERFACE_TRANSP, "Encapsulated Interface Transport" },
    { 0,                      NULL }
};

/* Translate exception code to string */
static const value_string exception_code_vals[] = {
    { ILLEGAL_FUNCTION,    "Illegal function" },
    { ILLEGAL_ADDRESS,     "Illegal data address" },
    { ILLEGAL_VALUE,       "Illegal data value" },
    { SLAVE_FAILURE,       "Slave device failure" },
    { ACKNOWLEDGE,         "Acknowledge" },
    { SLAVE_BUSY,          "Slave device busy" },
    { MEMORY_ERR,          "Memory parity error" },
    { GATEWAY_UNAVAILABLE, "Gateway path unavailable" },
    { GATEWAY_TRGT_FAIL,   "Gateway target device failed to respond" },
    { 0,                    NULL }
};

/* Translate Modbus Encapsulation Interface (MEI) code to string */
static const value_string encap_interface_code_vals[] = {
    { CANOPEN_REQ_RESP, "CANopen Request/Response " },
    { READ_DEVICE_ID,   "Read Device Identification" },
    { 0,                NULL }
};

/* Translate Modbus Diagnostic subfunction code to string */
static const value_string diagnostic_code_vals[] = {
    { RETURN_QUERY_DATA,                "Return Query Data" },
    { RESTART_COMMUNICATION_OPTION,     "Restart Communications Option" },
    { RETURN_DIAGNOSTIC_REGISTER,       "Return Diagnostic Register" },
    { CHANGE_ASCII_INPUT_DELIMITER,     "Change ASCII Input Delimiter" },
    { FORCE_LISTEN_ONLY_MODE,           "Force Listen Only Mode" },
    { CLEAR_COUNTERS_AND_DIAG_REG,      "Clear Counters and Diagnostic Register" },
    { RETURN_BUS_MESSAGE_COUNT,         "Return Bus Message Count" },
    { RETURN_BUS_COMM_ERROR_COUNT,      "Return Bus Communication Error Count" },
    { RETURN_BUS_EXCEPTION_ERROR_COUNT, "Return Bus Exception Error Count" },
    { RETURN_SLAVE_MESSAGE_COUNT,       "Return Slave Message Count" },
    { RETURN_SLAVE_NO_RESPONSE_COUNT,   "Return Slave No Response Count" },
    { RETURN_SLAVE_NAK_COUNT,           "Return Slave NAK Count" },
    { RETURN_SLAVE_BUSY_COUNT,          "Return Slave Busy Count" },
    { RETURN_BUS_CHAR_OVERRUN_COUNT,    "Return Bus Character Overrun Count" },
    { CLEAR_OVERRUN_COUNTER_AND_FLAG,   "Clear Overrun Counter and Flag" },
    { 0,                                NULL }
};

static const value_string diagnostic_restart_communication_option_vals[] = {
    { 0,        "Leave Log" },
    { 0xFF,     "Clear Log" },
    { 0,        NULL }
};

/* Translate read device code to string */
static const value_string read_device_id_vals[] = {
    { 1,        "Basic Device Identification" },
    { 2,        "Regular Device Identification"  },
    { 3,        "Extended Device Identification"  },
    { 4,        "Specific Identification Object"  },

    { 0,        NULL             }
};

/* Translate read device code to string */
static const value_string object_id_vals[] = {
    { 0,        "VendorName" },
    { 1,        "ProductCode" },
    { 2,        "MajorMinorRevision"  },
    { 3,        "VendorURL"  },
    { 4,        "ProductName"  },
    { 5,        "ModelName"  },
    { 6,        "UserApplicationName"  },

    { 0,        NULL             }
};

static const value_string conformity_level_vals[] = {
    { 0x01,     "Basic Device Identification (stream)" },
    { 0x02,     "Regular Device Identification (stream)"  },
    { 0x03,     "Extended Device Identification (stream)"  },
    { 0x81,     "Basic Device Identification (stream and individual)" },
    { 0x82,     "Regular Device Identification (stream and individual)"  },
    { 0x83,     "Extended Device Identification (stream and individual)"  },

    { 0,        NULL             }
};

static const enum_val_t mbus_register_format[] = {
  { "UINT16     ", "UINT16     ",  MBTCP_PREF_REGISTER_FORMAT_UINT16  },
  { "UINT32     ", "UINT32     ",  MBTCP_PREF_REGISTER_FORMAT_UINT32  },
  { "IEEE FLT   ", "IEEE FLT   ",  MBTCP_PREF_REGISTER_FORMAT_IEEE_FLOAT  },
  { "MODICON FLT", "MODICON FLT",  MBTCP_PREF_REGISTER_FORMAT_MODICON_FLOAT  },
  { NULL, NULL, 0 }
};

static const enum_val_t mbus_register_addr_type[] = {
  { "RAW            ", "RAW            ",  MBTCP_PREF_REGISTER_ADDR_RAW  },
  { "MODICON 5-DIGIT", "MODICON 5-DIGIT",  MBTCP_PREF_REGISTER_ADDR_MOD5  },
  { "MODICON 6-DIGIT", "MODICON 6-DIGIT",  MBTCP_PREF_REGISTER_ADDR_MOD6  },
  { NULL, NULL, 0 }
};

/* Code to dissect Modbus/TCP packets */
static int
dissect_mbtcp_pdu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
/* Set up structures needed to add the protocol subtree and manage it */
    proto_item    *mi;
    proto_tree    *mbtcp_tree;
    int           offset, packet_type;
    tvbuff_t      *next_tvb;
    const char    *func_string = "";
    const char    *pkt_type_str = "";
    const char    *err_str = "";
    guint16       transaction_id, protocol_id, len;
    guint8        unit_id, function_code, exception_code, subfunction_code;
    void          *p_save_proto_data;
    modbus_request_info_t *request_info;

    /* Make entries in Protocol column on summary display */
    col_set_str(pinfo->cinfo, COL_PROTOCOL, "Modbus/TCP");
    col_clear(pinfo->cinfo, COL_INFO);

    transaction_id = tvb_get_ntohs(tvb, 0);
    protocol_id = tvb_get_ntohs(tvb, 2);
    len = tvb_get_ntohs(tvb, 4);

    unit_id = tvb_get_guint8(tvb, 6);
    function_code = tvb_get_guint8(tvb, 7) & 0x7F;

    /* Make entries in Info column on summary display */
    offset = 0;

    /* Find exception - last bit set in function code */
    if (tvb_get_guint8(tvb, 7) & 0x80) {
        exception_code = tvb_get_guint8(tvb, offset + 8);
    }
    else {
        exception_code = 0;
    }

    if ((function_code == ENCAP_INTERFACE_TRANSP) && (exception_code == 0))  {
        func_string = val_to_str_const(tvb_get_guint8(tvb, offset + 8), encap_interface_code_vals, "Encapsulated Interface Transport");
        subfunction_code = 1;
    }
    else if ((function_code == DIAGNOSTICS) && (exception_code == 0))  {
        func_string = val_to_str_const(tvb_get_ntohs(tvb, offset + 8), diagnostic_code_vals, "Diagnostics");
        subfunction_code = 1;
    }
    else {
        func_string = val_to_str(function_code, function_code_vals, "Unknown function (%d)");
        subfunction_code = 0;
    }

    /* "Request" or "Response" */
    packet_type = classify_mbtcp_packet(pinfo);

    switch ( packet_type ) {
        case QUERY_PACKET :
            pkt_type_str="Query";
            break;
        case RESPONSE_PACKET :
            pkt_type_str="Response";
            break;
        case CANNOT_CLASSIFY :
            err_str="Unable to classify as query or response.";
            pkt_type_str="unknown";
            break;
        default :
            break;
    }
    if ( exception_code != 0 )
        err_str="Exception returned ";

    if (subfunction_code == 0) {
        if (strlen(err_str) > 0) {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Trans: %5u; Unit: %3u, Func: %3u: %s. %s",
                    pkt_type_str, transaction_id, unit_id,
                    function_code, func_string, err_str);
        }
        else {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Trans: %5u; Unit: %3u, Func: %3u: %s",
                    pkt_type_str, transaction_id, unit_id,
                    function_code, func_string);
        }
    }
    else {
        if (strlen(err_str) > 0) {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Trans: %5u; Unit: %3u, Func: %3u/%3u: %s. %s",
                    pkt_type_str, transaction_id, unit_id,
                    function_code, subfunction_code, func_string, err_str);
        }
        else {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Trans: %5u; Unit: %3u, Func: %3u/%3u: %s",
                    pkt_type_str, transaction_id, unit_id,
                    function_code, subfunction_code, func_string);
        }
    }

    mi = proto_tree_add_protocol_format(tree, proto_mbtcp, tvb, offset,
            len+6, "Modbus/TCP");
    mbtcp_tree = proto_item_add_subtree(mi, ett_mbtcp);

    /* Add items to protocol tree specific to Modbus/TCP */
    proto_tree_add_uint(mbtcp_tree, hf_mbtcp_transid, tvb, offset, 2, transaction_id);
    proto_tree_add_uint(mbtcp_tree, hf_mbtcp_protid, tvb, offset + 2, 2, protocol_id);
    proto_tree_add_uint(mbtcp_tree, hf_mbtcp_len, tvb, offset + 4, 2, len);
    proto_tree_add_uint(mbtcp_tree, hf_mbtcp_unitid, tvb, offset + 6, 1, unit_id);

    /* dissect the Modbus PDU */
    next_tvb = tvb_new_subset( tvb, offset+7, len-1, len-1);

    /* keep existing context */
    p_save_proto_data = p_get_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0 );
    p_remove_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0);

    /* Create enough context for Modbus dissector */
    request_info = wmem_new(wmem_packet_scope(), modbus_request_info_t);
    request_info->packet_type = (guint8)packet_type;
    request_info->register_addr_type = (guint8)global_mbus_tcp_register_addr_type;
    request_info->register_format = (guint8)global_mbus_tcp_register_format;
    p_add_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0, request_info);

    /* Continue with dissection of Modbus data payload following Modbus/TCP frame */
    if( tvb_length_remaining(tvb, offset) > 0 )
        call_dissector(modbus_handle, next_tvb, pinfo, tree);

    p_remove_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0);
    p_add_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0, p_save_proto_data);
    return tvb_length(tvb);
}

/* Code to dissect Modbus RTU over TCP packets */
static int
dissect_mbrtu_pdu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
/* Set up structures needed to add the protocol subtree and manage it */
    proto_item    *mi, *crc_item;
    proto_tree    *mbrtu_tree;
    gint           offset, packet_type;
    tvbuff_t      *next_tvb;
    const char    *func_string = "";
    const char    *pkt_type_str = "";
    const char    *err_str = "";
    guint16       len, crc16, calc_crc16;
    guint8        unit_id, function_code, exception_code, subfunction_code;
    void          *p_save_proto_data;
    modbus_request_info_t *request_info;

    /* Make entries in Protocol column on summary display */
    col_set_str(pinfo->cinfo, COL_PROTOCOL, "Modbus RTU");
    col_clear(pinfo->cinfo, COL_INFO);

    len = tvb_length(tvb);

    unit_id = tvb_get_guint8(tvb, 0);
    function_code = tvb_get_guint8(tvb, 1) & 0x7F;
    crc16 = tvb_get_ntohs(tvb, len-2);

    /* Make entries in Info column on summary display */
    offset = 0;

    /* Find exception - last bit set in function code */
    if (tvb_get_guint8(tvb, 1) & 0x80) {
        exception_code = tvb_get_guint8(tvb, offset + 2);
    }
    else {
        exception_code = 0;
    }

    if ((function_code == ENCAP_INTERFACE_TRANSP) && (exception_code == 0))  {
        func_string = val_to_str_const(tvb_get_guint8(tvb, offset + 2), encap_interface_code_vals, "Encapsulated Interface Transport");
        subfunction_code = 1;
    }
    else if ((function_code == DIAGNOSTICS) && (exception_code == 0))  {
        func_string = val_to_str_const(tvb_get_ntohs(tvb, offset + 2), diagnostic_code_vals, "Diagnostics");
        subfunction_code = 1;
    }
    else {
        func_string = val_to_str(function_code, function_code_vals, "Unknown function (%d)");
        subfunction_code = 0;
    }

    /* "Request" or "Response" */
    packet_type = classify_mbrtu_packet(pinfo, tvb);

    switch ( packet_type ) {
        case QUERY_PACKET :
            pkt_type_str="Query";
            break;
        case RESPONSE_PACKET :
            pkt_type_str="Response";
            break;
        case CANNOT_CLASSIFY :
            err_str="Unable to classify as query or response.";
            pkt_type_str="unknown";
            break;
        default :
            break;
    }
    if ( exception_code != 0 )
        err_str="Exception returned ";

    if (subfunction_code == 0) {
        if (strlen(err_str) > 0) {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Unit: %3u, Func: %3u: %s. %s",
                    pkt_type_str, unit_id,
                    function_code, func_string, err_str);
        }
        else {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Unit: %3u, Func: %3u: %s",
                    pkt_type_str, unit_id,
                    function_code, func_string);
        }
    }
    else {
        if (strlen(err_str) > 0) {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Unit: %3u, Func: %3u/%3u: %s. %s",
                    pkt_type_str, unit_id,
                    function_code, subfunction_code, func_string, err_str);
        }
        else {
            col_add_fstr(pinfo->cinfo, COL_INFO,
                    "%8s: Unit: %3u, Func: %3u/%3u: %s",
                    pkt_type_str, unit_id,
                    function_code, subfunction_code, func_string);
        }
    }

    mi = proto_tree_add_protocol_format(tree, proto_mbrtu, tvb, offset,
            len, "Modbus RTU");
    mbrtu_tree = proto_item_add_subtree(mi, ett_mbrtu);

    /* Add items to protocol tree specific to Modbus RTU */
    proto_tree_add_uint(mbrtu_tree, hf_mbrtu_unitid, tvb, offset, 1, unit_id);
    crc_item = proto_tree_add_uint(mbrtu_tree, hf_mbrtu_crc16, tvb, len-2, 2, crc16);

    /* CRC validation */
    if (mbrtu_crc)
    {
        calc_crc16 = crc16_plain_tvb_offset_seed(tvb, offset, len-2, 0xFFFF);
        if (g_htons(calc_crc16) != crc16)
            expert_add_info_format(pinfo, crc_item, &ei_mbrtu_crc16_incorrect, "Incorrect CRC - should be 0x%04x", g_htons(calc_crc16));
    }

    /* make sure to ignore the CRC-16 footer bytes */
    len = len - 2;

    /* dissect the Modbus PDU                      */
    next_tvb = tvb_new_subset( tvb, offset+1, len-1, len-1);

    /* keep existing context */
    p_save_proto_data = p_get_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0 );
    p_remove_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0);

    /* Create enough context for Modbus dissector */
    request_info = wmem_new(wmem_packet_scope(), modbus_request_info_t);
    request_info->packet_type = (guint8)packet_type;
    request_info->register_addr_type = (guint8)global_mbus_rtu_register_addr_type;
    request_info->register_format = (guint8)global_mbus_rtu_register_format;
    p_add_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0, request_info);

    /* Continue with dissection of Modbus data payload following Modbus RTU frame */
    if( tvb_length_remaining(tvb, offset) > 0 )
        call_dissector(modbus_handle, next_tvb, pinfo, tree);

    p_remove_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0);
    p_add_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0, p_save_proto_data);
    return tvb_length(tvb);
}


/* Return length of Modbus/TCP message */
static guint
get_mbtcp_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset)
{
    guint16 plen;

    /*
     * Get the length of the data from the encapsulation header.
     */
    plen = tvb_get_ntohs(tvb, offset + 4);

    /*
     * That length doesn't include the encapsulation header itself;
     * add that in.
     */
    return plen + 6;
}

/* Return length of Modbus RTU over TCP message */
static guint
get_mbrtu_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset _U_)
{

    /* Modbus/TCP frames include a "length" word in each message; Modbus RTU over TCP does not, so don't attempt to get one */
    return tvb_length(tvb);
}


/* Code to dissect Modbus/TCP messages */
static int
dissect_mbtcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
{

    /* Make sure there's at least enough data to determine it's a Modbus TCP packet */
    if (!tvb_bytes_exist(tvb, 0, 8))
        return 0;

    /* check that it actually looks like Modbus/TCP */
    /* protocol id == 0 */
    if(tvb_get_ntohs(tvb, 2) != 0 ){
        return 0;
    }
    /* length is at least 2 (unit_id + function_code) */
    if(tvb_get_ntohs(tvb, 4) < 2 ){
        return 0;
    }

    /* build up protocol tree and iterate over multiple packets */
    tcp_dissect_pdus(tvb, pinfo, tree, mbtcp_desegment, 6,
                     get_mbtcp_pdu_len, dissect_mbtcp_pdu, data);

    return tvb_length(tvb);
}

/* Code to dissect Modbus RTU over TCP messages */
static int
dissect_mbrtu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
{

    /* Make sure there's at least enough data to determine it's a Modbus packet */
    if (!tvb_bytes_exist(tvb, 0, 6))
        return 0;

    /* For Modbus RTU mode, confirm that the first byte is a valid address (non-zero), */
    /* so we can eliminate false-posititves on Modbus TCP messages loaded as RTU       */
    if(tvb_get_guint8(tvb, 0) == 0 )
        return 0;

    /* build up protocol tree and iterate over multiple packets */
    tcp_dissect_pdus(tvb, pinfo, tree, mbrtu_desegment, 6,
                     get_mbrtu_pdu_len, dissect_mbrtu_pdu, data);

    return tvb_length(tvb);
}


/* Code to allow further dissection of Modbus data payload */
/* Common to both Modbus/TCP and Modbus RTU dissectors     */
static void
dissect_modbus_data(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, guint8 function_code,
                    gint payload_start, gint payload_len, guint8 register_format)
{
    gint reported_len, data_offset, reg_num = 0;
    guint16 data16, modflt_lo, modflt_hi;
    guint32 data32, modflt_comb;
    gfloat data_float, modfloat;
    proto_item    *register_item = NULL;
    tvbuff_t *next_tvb;

    reported_len = tvb_reported_length_remaining(tvb, payload_start);
    data_offset = 0;

    if ( payload_start < 0 || ( payload_len + payload_start ) == 0 )
        return;

    /* If calculated length from remaining tvb data != bytes in packet, do not attempt to decode      */
    if ( payload_len != reported_len ) {
        proto_tree_add_item(tree, hf_modbus_data, tvb, payload_start, reported_len, ENC_NA);
        return;
    }

    /* If data type of payload is Holding or Input registers */
    /* AND */
    /* if payload length is not a multiple of 4, don't attempt to decode anything in 32-bit format */
    if ((function_code == READ_HOLDING_REGS) || (function_code == READ_INPUT_REGS) || (function_code == WRITE_MULT_REGS)) {
        if ((payload_len % 4 != 0) && ( (register_format == MBTCP_PREF_REGISTER_FORMAT_UINT32) ||
            (register_format == MBTCP_PREF_REGISTER_FORMAT_IEEE_FLOAT) ||
            (register_format == MBTCP_PREF_REGISTER_FORMAT_MODICON_FLOAT) ) ) {
            register_item = proto_tree_add_item(tree, hf_modbus_data, tvb, payload_start, payload_len, ENC_NA);
            expert_add_info(pinfo, register_item, &ei_modbus_data_decode);
            return;
        }
    }

    /* Build a new tvb containing just the data payload   */
    next_tvb = tvb_new_subset(tvb, payload_start, payload_len, reported_len);

    switch ( function_code ) {

        case READ_HOLDING_REGS:
        case READ_INPUT_REGS:
        case WRITE_MULT_REGS:
            while (data_offset < payload_len) {
                /* Use "Preferences" options to determine decoding format of register data, as no format is implied by the protocol itself. */
                /* Based on a standard register size of 16-bits, use decoding format preference to step through each register and display  */
                /* it in an appropriate fashion. */
                switch (register_format) {
                    case MBTCP_PREF_REGISTER_FORMAT_UINT16: /* Standard-size unsigned integer 16-bit register */
                        data16 = tvb_get_ntohs(next_tvb, data_offset);
                        register_item = proto_tree_add_uint(tree, hf_modbus_reg_uint16, next_tvb, data_offset, 2, data16);
                        proto_item_set_text(register_item, "Register %u (UINT16): %u", reg_num, data16);

                        data_offset += 2;
                        reg_num += 1;
                        break;
                    case MBTCP_PREF_REGISTER_FORMAT_UINT32: /* Double-size unsigned integer 2 x 16-bit registers */
                        data32 = tvb_get_ntohl(next_tvb, data_offset);
                        register_item = proto_tree_add_uint(tree, hf_modbus_reg_uint32, next_tvb, data_offset, 4, data32);
                        proto_item_set_text(register_item, "Register %u (UINT32): %u", reg_num, data32);

                        data_offset += 4;
                        reg_num += 2;
                        break;
                    case MBTCP_PREF_REGISTER_FORMAT_IEEE_FLOAT: /* IEEE Floating Point, 2 x 16-bit registers */
                        data_float = tvb_get_ntohieee_float(next_tvb, data_offset);
                        register_item = proto_tree_add_float(tree, hf_modbus_reg_ieee_float, next_tvb, data_offset, 4, data_float);
                        proto_item_set_text(register_item, "Register %u (IEEE Float): %f", reg_num, data_float);

                        data_offset += 4;
                        reg_num += 2;
                        break;
                    case MBTCP_PREF_REGISTER_FORMAT_MODICON_FLOAT: /* Modicon Floating Point (word-swap), 2 x 16-bit registers */
                        /* Modicon-style Floating Point values are stored in reverse-word order.                     */
                        /* ie: a standard IEEE float value 59.991459 is equal to 0x426ff741                          */
                        /*     while the Modicon equivalent to this value is 0xf741426f                              */
                        /* To re-assemble a proper IEEE float, we must retrieve the 2 x 16-bit words, bit-shift the  */
                        /* "hi" component by 16-bits and then OR them together into a combined 32-bit int.           */
                        /* Following that operation, use some memcpy magic to copy the 4 raw data bytes from the     */
                        /* 32-bit integer into a standard float.  Not sure if there is a cleaner way possible using  */
                        /* the Wireshark libaries, but this seems to work OK.                                        */

                        modflt_lo = tvb_get_ntohs(next_tvb, data_offset);
                        modflt_hi = tvb_get_ntohs(next_tvb, data_offset+2);
                        modflt_comb = (guint32)(modflt_hi<<16) | modflt_lo;
                        memcpy(&modfloat, &modflt_comb, 4);

                        register_item = proto_tree_add_float(tree, hf_modbus_reg_modicon_float, next_tvb, data_offset, 4, modfloat);
                        proto_item_set_text(register_item, "Register %u (Modicon Float): %f", reg_num, modfloat);

                        data_offset += 4;
                        reg_num += 2;
                        break;
                    default:
                        /* Avoid any chance of an infinite loop */
                        data_offset = payload_len;
                        break;
                    } /* register format switch */

                } /* while loop */

                break;

        default:
            if ( ! dissector_try_string(modbus_data_dissector_table, "data", next_tvb, pinfo, tree, NULL) )
                proto_tree_add_item(tree, hf_modbus_data, tvb, payload_start, payload_len, ENC_NA);
            break;
        }
}

/* Code to actually dissect the packets */
static int
dissect_modbus(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data _U_)
{
    proto_tree    *modbus_tree, *group_tree, *event_tree,
                  *event_item_tree, *device_objects_tree,
                  *device_objects_item_tree;
    proto_item    *mi, *mf, *me, *mei, *doe, *doie;
    int           offset = 0, group_offset;
    gint          payload_start, payload_len, event_index,
                  ii, byte_cnt, len, num_objects, object_index,
                  object_len;
    guint32       group_byte_cnt, group_word_cnt;
    guint8        function_code, exception_code, mei_code, event_code, object_type;
    guint8        packet_type, register_format; /*register_addr_type*/
    guint16       diagnostic_code;
    modbus_request_info_t *request_info;

    len = tvb_length_remaining(tvb, 0);

    /* If the packet is zero-length, we should not attempt to dissect any further */
    if (len == 0)
        return 0;

    function_code = tvb_get_guint8(tvb, offset) & 0x7F;

    /* Find exception - last bit set in function code */
    if (tvb_get_guint8(tvb, offset) & 0x80 ) {
        exception_code = tvb_get_guint8(tvb, offset+1);
    }
    else {
        exception_code = 0;
    }

    /* See if we have any context */
    request_info = (modbus_request_info_t *)p_get_proto_data(wmem_file_scope(), pinfo, proto_modbus, 0 );
    if (request_info != NULL)
    {
        packet_type = request_info->packet_type;
        register_format = request_info->register_format;
        /*register_addr_type = request_info->register_addr_type;*/
    }
    else
    {
        /* Default to a response packet to at least attempt to decode a good chunk of data */
        packet_type = RESPONSE_PACKET;
        register_format = MBTCP_PREF_REGISTER_FORMAT_UINT16;
       /* register_addr_type = MBTCP_PREF_REGISTER_ADDR_RAW;*/
    }

    /* Add items to protocol tree specific to Modbus generic */
    mf = proto_tree_add_text(tree, tvb, offset, len, "Modbus");
    modbus_tree = proto_item_add_subtree(mf, ett_modbus_hdr);

    mi = proto_tree_add_uint(modbus_tree, hf_mbtcp_functioncode, tvb, offset, 1,
                             function_code);

    payload_start = offset + 1;
    payload_len = len - 1;
    if (exception_code != 0) {
        proto_item_set_text(mi, "Function %u:  %s.  Exception: %s",
                            function_code,
                            val_to_str_const(function_code, function_code_vals, "Unknown Function"),
                            val_to_str(exception_code,
                                       exception_code_vals,
                                       "Unknown Exception Code (%u)"));
        proto_tree_add_uint(modbus_tree, hf_modbus_exceptioncode, tvb, payload_start, 1,
                            exception_code);
    }
    else {
        switch (function_code) {

            case READ_COILS:
            case READ_DISCRETE_INPUTS:

                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1, byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt, register_format);
                }
                break;

            case READ_HOLDING_REGS:
            case READ_INPUT_REGS:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1, byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt, register_format);
                }
                break;

            case WRITE_SINGLE_COIL:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 1, register_format);
                    proto_tree_add_text(modbus_tree, tvb, payload_start + 3, 1, "Padding");
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 1, register_format);
                    proto_tree_add_text(modbus_tree, tvb, payload_start + 3, 1, "Padding");
                }
                break;

            case WRITE_SINGLE_REG:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 2, register_format);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 2, register_format);
                }
                break;

            case READ_EXCEPT_STAT:
                if (packet_type == RESPONSE_PACKET)
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start, 1, register_format);
                break;

            case DIAGNOSTICS:
                if ((packet_type == QUERY_PACKET) || (packet_type == RESPONSE_PACKET)) {
                    diagnostic_code = tvb_get_ntohs(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_diag_sf, tvb, payload_start, 2, diagnostic_code);
                    switch(diagnostic_code)
                    {
                        case RETURN_QUERY_DATA:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    proto_tree_add_item(modbus_tree, hf_modbus_diag_return_query_data_request, tvb, payload_start+2, payload_len-2, ENC_NA);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                if (payload_len > 2)
                                    proto_tree_add_item(modbus_tree, hf_modbus_diag_return_query_data_echo, tvb, payload_start+2, payload_len-2, ENC_NA);
                            }
                            break;
                        case RESTART_COMMUNICATION_OPTION:
                            proto_tree_add_item(modbus_tree, hf_modbus_diag_restart_communication_option, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            break;
                        case RETURN_DIAGNOSTIC_REGISTER:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_diag_register, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case CHANGE_ASCII_INPUT_DELIMITER:
                            proto_tree_add_item(modbus_tree, hf_modbus_diag_ascii_input_delimiter, tvb, payload_start+2, 1, ENC_BIG_ENDIAN);
                            break;
                        case RETURN_BUS_MESSAGE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_bus_message_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_BUS_COMM_ERROR_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_bus_comm_error_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_BUS_EXCEPTION_ERROR_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_bus_exception_error_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_SLAVE_MESSAGE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_slave_message_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_SLAVE_NO_RESPONSE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_no_slave_response_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_SLAVE_NAK_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_slave_nak_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_SLAVE_BUSY_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_slave_busy_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case RETURN_BUS_CHAR_OVERRUN_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            }
                            else if (packet_type == RESPONSE_PACKET) {
                                proto_tree_add_item(modbus_tree, hf_modbus_diag_return_bus_char_overrun_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                            }
                            break;
                        case CLEAR_OVERRUN_COUNTER_AND_FLAG:
                        case FORCE_LISTEN_ONLY_MODE:
                        case CLEAR_COUNTERS_AND_DIAG_REG:
                        default:
                            if (payload_len > 2)
                                dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2, register_format);
                            break;
                    }
                }
                break;

            case GET_COMM_EVENT_CTRS:
                if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_status, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_event_count, tvb, payload_start+2, 2, ENC_BIG_ENDIAN);
                }
                break;

            case GET_COMM_EVENT_LOG:
                if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1, byte_cnt);
                    proto_tree_add_item(modbus_tree, hf_modbus_status, tvb, payload_start+1, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_event_count, tvb, payload_start+3, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_message_count, tvb, payload_start+5, 2, ENC_BIG_ENDIAN);
                    if (byte_cnt-6 > 0) {
                        byte_cnt -= 6;
                        event_index = 0;
                        me = proto_tree_add_text(modbus_tree, tvb, payload_start+7, byte_cnt, "Events");
                        event_tree = proto_item_add_subtree(me, ett_events);
                        while (byte_cnt > 0) {
                            event_code = tvb_get_guint8(tvb, payload_start+7+event_index);
                            if (event_code == 0) {
                                proto_tree_add_text(event_tree, tvb, payload_start+7+event_index, 1, "Initiated Communication Restart");
                            }
                            else if (event_code == 4) {
                                proto_tree_add_text(event_tree, tvb, payload_start+7+event_index, 1, "Entered Listen Only Mode");
                            }
                            else if (event_code & REMOTE_DEVICE_RECV_EVENT_MASK) {
                                mei = proto_tree_add_text(event_tree, tvb, payload_start+7+event_index, 1, "Receive Event: 0x%02X", event_code);
                                event_item_tree = proto_item_add_subtree(mei, ett_events_recv);

                                /* add subtrees to describe each event bit */
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_comm_err,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_char_over,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_lo_mode,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_broadcast,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                            }
                            else if ((event_code & REMOTE_DEVICE_SEND_EVENT_MASK) == REMOTE_DEVICE_SEND_EVENT_VALUE) {
                                mei = proto_tree_add_text(event_tree, tvb, payload_start+7+event_index, 1, "Send Event: 0x%02X", event_code);
                                event_item_tree = proto_item_add_subtree(mei, ett_events_send);

                                /* add subtrees to describe each event bit */
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_read_ex,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_abort_ex,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_busy_ex,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_nak_ex,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_write_timeout,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_lo_mode,
                                  tvb, payload_start+7+event_index, 1, ENC_LITTLE_ENDIAN );
                            }
                            else {
                                proto_tree_add_text(event_tree, tvb, payload_start+7+event_index, 1, "Unknown Event");
                            }

                            byte_cnt--;
                            event_index++;
                        }
                    }
                }
                break;

            case WRITE_MULT_COILS:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start + 4);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start + 4, 1,
                            byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 5, byte_cnt, register_format);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                }
                break;

            case WRITE_MULT_REGS:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start + 4);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start + 4, 1,
                            byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 5, byte_cnt, register_format);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                }
                break;

            case READ_FILE_RECORD:
                if (packet_type == QUERY_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1,
                            byte_cnt);

                    /* add subtrees to describe each group of packet */
                    group_offset = payload_start + 1;
                    for (ii = 0; ii < byte_cnt / 7; ii++) {
                        mi = proto_tree_add_text( modbus_tree, tvb, group_offset, 7,
                                "Group %u", ii);
                        group_tree = proto_item_add_subtree(mi, ett_group_hdr);
                        proto_tree_add_item(group_tree, hf_modbus_reftype, tvb, group_offset, 1, ENC_BIG_ENDIAN);
                        proto_tree_add_item(group_tree, hf_modbus_lreference, tvb, group_offset + 1, 4, ENC_BIG_ENDIAN);
                        proto_tree_add_item(group_tree, hf_modbus_wordcnt, tvb, group_offset + 5, 2, ENC_BIG_ENDIAN);
                        group_offset += 7;
                    }
                }
                else if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1,
                            byte_cnt);

                    /* add subtrees to describe each group of packet */
                    group_offset = payload_start + 1;
                    ii = 0;
                    while (byte_cnt > 0) {
                        group_byte_cnt = (guint32)tvb_get_guint8(tvb, group_offset);
                        mi = proto_tree_add_text( modbus_tree, tvb, group_offset, group_byte_cnt + 1,
                                "Group %u", ii);
                        group_tree = proto_item_add_subtree(mi, ett_group_hdr);
                        proto_tree_add_uint(group_tree, hf_modbus_bytecnt, tvb, group_offset, 1,
                                group_byte_cnt);
                        proto_tree_add_item(group_tree, hf_modbus_reftype, tvb, group_offset + 1, 1, ENC_BIG_ENDIAN);
                        dissect_modbus_data(tvb, pinfo, group_tree, function_code, group_offset + 2, group_byte_cnt - 1, register_format);
                        group_offset += (group_byte_cnt + 1);
                        byte_cnt -= (group_byte_cnt + 1);
                        ii++;
                    }
                }
                break;

            case WRITE_FILE_RECORD:
                if ((packet_type == QUERY_PACKET) || (packet_type == RESPONSE_PACKET)) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1,
                            byte_cnt);

                    /* add subtrees to describe each group of packet */
                    group_offset = payload_start + 1;
                    ii = 0;
                    while (byte_cnt > 0) {
                        group_word_cnt = tvb_get_ntohs(tvb, group_offset + 5);
                        group_byte_cnt = (2 * group_word_cnt) + 7;
                        mi = proto_tree_add_text( modbus_tree, tvb, group_offset,
                                group_byte_cnt, "Group %u", ii);
                        group_tree = proto_item_add_subtree(mi, ett_group_hdr);
                        proto_tree_add_item(group_tree, hf_modbus_reftype, tvb, group_offset, 1, ENC_BIG_ENDIAN);
                        proto_tree_add_item(group_tree, hf_modbus_lreference, tvb, group_offset + 1, 4, ENC_BIG_ENDIAN);
                        proto_tree_add_uint(group_tree, hf_modbus_wordcnt, tvb, group_offset + 5, 2,
                                group_word_cnt);
                        dissect_modbus_data(tvb, pinfo, group_tree, function_code, group_offset + 7, group_byte_cnt - 7, register_format);
                        group_offset += group_byte_cnt;
                        byte_cnt -= group_byte_cnt;
                        ii++;
                    }
                }
                break;

            case MASK_WRITE_REG:
                if ((packet_type == QUERY_PACKET) || (packet_type == RESPONSE_PACKET)) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_andmask, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_ormask, tvb, payload_start + 4, 2, ENC_BIG_ENDIAN);
                }
                break;

            case READ_WRITE_REG:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_readref, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_readwordcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_writeref, tvb, payload_start + 4, 2, ENC_BIG_ENDIAN);
                    proto_tree_add_item(modbus_tree, hf_modbus_writewordcnt, tvb, payload_start + 6, 2, ENC_BIG_ENDIAN);
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start + 8);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start + 8, 1,
                            byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 9, byte_cnt, register_format);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_guint8(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_bytecnt, tvb, payload_start, 1,
                            byte_cnt);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt, register_format);
                }
                break;

            case READ_FIFO_QUEUE:
                if (packet_type == QUERY_PACKET)
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, ENC_BIG_ENDIAN);
                else if (packet_type == RESPONSE_PACKET) {
                    byte_cnt = (guint32)tvb_get_ntohs(tvb, payload_start);
                    proto_tree_add_uint(modbus_tree, hf_modbus_lbytecnt, tvb, payload_start, 2,
                            byte_cnt);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, ENC_BIG_ENDIAN);
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start + 4, byte_cnt - 2, register_format);
                }
                break;

            case ENCAP_INTERFACE_TRANSP:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_mei, tvb, payload_start, 1, ENC_BIG_ENDIAN);
                    mei_code = tvb_get_guint8(tvb, payload_start);
                    switch (mei_code)
                    {
                        case READ_DEVICE_ID:
                            proto_tree_add_item(modbus_tree, hf_modbus_read_device_id, tvb, payload_start+1, 1, ENC_BIG_ENDIAN);
                            proto_tree_add_item(modbus_tree, hf_modbus_object_id, tvb, payload_start+2, 1, ENC_BIG_ENDIAN);
                            break;

                        case CANOPEN_REQ_RESP:
                            /* CANopen protocol not part of the Modbus/TCP specification */
                        default:
                            if (payload_len > 1)
                                dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start, payload_len-1, register_format);
                                break;
                    }
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_mei, tvb, payload_start, 1, ENC_BIG_ENDIAN);
                    mei_code = tvb_get_guint8(tvb, payload_start);
                    switch (mei_code)
                    {
                        case READ_DEVICE_ID:
                            proto_tree_add_item(modbus_tree, hf_modbus_read_device_id, tvb, payload_start+1, 1, ENC_BIG_ENDIAN);
                            proto_tree_add_item(modbus_tree, hf_modbus_conformity_level, tvb, payload_start+2, 1, ENC_BIG_ENDIAN);
                            proto_tree_add_item(modbus_tree, hf_modbus_more_follows, tvb, payload_start+3, 1, ENC_BIG_ENDIAN);
                            proto_tree_add_item(modbus_tree, hf_modbus_next_object_id, tvb, payload_start+4, 1, ENC_BIG_ENDIAN);
                            num_objects = tvb_get_guint8(tvb, payload_start+5);
                            proto_tree_add_uint(modbus_tree, hf_modbus_num_objects, tvb, payload_start+5, 1, num_objects);
                            doe = proto_tree_add_text(modbus_tree, tvb, payload_start+6, payload_len-6, "Objects");

                            object_index = 0;
                            for (ii = 0; ii < num_objects; ii++)
                            {
                                device_objects_tree = proto_item_add_subtree(doe, ett_device_id_objects);

                                /* add each "object item" as its own subtree */

                                /* compute length of object */
                                object_type = tvb_get_guint8(tvb, payload_start+6+object_index);
                                object_len = tvb_get_guint8(tvb, payload_start+6+object_index+1);

                                doie = proto_tree_add_text(device_objects_tree, tvb, payload_start+6+object_index, 2+object_len, "Object #%d", ii+1);
                                device_objects_item_tree = proto_item_add_subtree(doie, ett_device_id_object_items);

                                proto_tree_add_item(device_objects_item_tree, hf_modbus_object_id, tvb, payload_start+6+object_index, 1, ENC_BIG_ENDIAN);
                                object_index++;

                                proto_tree_add_uint(device_objects_item_tree, hf_modbus_list_object_len, tvb, payload_start+6+object_index, 1, object_len);
                                object_index++;

                                if (object_type < 7)
                                {
                                    proto_tree_add_item(device_objects_item_tree, hf_modbus_object_str_value, tvb, payload_start+6+object_index, object_len, ENC_ASCII|ENC_NA);
                                }
                                else
                                {
                                    if (object_len > 0)
                                        proto_tree_add_text(device_objects_item_tree, tvb, payload_start+6+object_index, object_len, "Object Value");
                                }
                                object_index += object_len;
                            }
                            break;

                        case CANOPEN_REQ_RESP:
                            /* CANopen protocol not part of the Modbus/TCP specification */
                        default:
                            if (payload_len > 1)
                                dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start, payload_len-1, register_format);
                                break;
                    }
                }
                break;

            case REPORT_SLAVE_ID:
            default:
                if (payload_len > 0)
                    dissect_modbus_data(tvb, pinfo, modbus_tree, function_code, payload_start, payload_len, register_format);
                break;
        }
    }

    return tvb_length(tvb);
}


/* Register the protocol with Wireshark */

void
proto_register_modbus(void)
{
    /* Modbus/TCP header fields */
    static hf_register_info mbtcp_hf[] = {
        { &hf_mbtcp_transid,
            { "Transaction Identifier",        "mbtcp.trans_id",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_mbtcp_protid,
            { "Protocol Identifier",           "mbtcp.prot_id",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_mbtcp_len,
            { "Length",                        "mbtcp.len",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_mbtcp_unitid,
            { "Unit Identifier",               "mbtcp.unit_id",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
    };

    static hf_register_info mbrtu_hf[] = {
        { &hf_mbrtu_unitid,
            { "Unit ID",                       "mbrtu.unit_id",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_mbrtu_crc16,
            { "CRC-16",                        "mbrtu.crc16",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
    };

    static ei_register_info mbrtu_ei[] = {
        { &ei_mbrtu_crc16_incorrect, { "mbrtu.crc16.incorrect", PI_CHECKSUM, PI_WARN, "Incorrect CRC", EXPFILL }},
    };

    static hf_register_info hf[] = {
        /* Modbus header fields */
        { &hf_mbtcp_functioncode,
            { "Function Code",                 "modbus.func_code",
            FT_UINT8, BASE_DEC, VALS(function_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reference,
            { "Reference Number",              "modbus.reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_lreference,
            { "Reference Number (32 bit)",     "modbus.reference_num_32",
            FT_UINT32, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reftype,
            { "Reference Type",                "modbus.reference_type",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_readref,
            { "Read Reference Number",         "modbus.read_reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_writeref,
            { "Write Reference Number",        "modbus.write_reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_wordcnt,
            { "Word Count",                    "modbus.word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_readwordcnt,
            { "Read Word Count",               "modbus.read_word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_writewordcnt,
            { "Write Word Count",              "modbus.write_word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_bitcnt,
            { "Bit Count",                     "modbus.bit_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_bytecnt,
            { "Byte Count",                    "modbus.byte_cnt",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_lbytecnt,
            { "Byte Count (16-bit)",           "modbus.byte_cnt_16",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_exceptioncode,
            { "Exception Code",                "modbus.exception_code",
            FT_UINT8, BASE_DEC, VALS(exception_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_sf,
            { "Diagnostic Code",               "modbus.diagnostic_code",
            FT_UINT16, BASE_DEC, VALS(diagnostic_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_query_data_request,
            { "Request Data",                  "modbus.diagnostic.return_query_data.request",
            FT_BYTES, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_query_data_echo,
            { "Echo Data",                  "modbus.diagnostic.return_query_data.echo",
            FT_BYTES, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_restart_communication_option,
            { "Restart Communication Option", "modbus.diagnostic.restart_communication_option",
            FT_UINT16, BASE_HEX, VALS(diagnostic_restart_communication_option_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_diag_register,
            { "Diagnostic Register Contents", "modbus.diagnostic.return_diag_register",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_ascii_input_delimiter,
            { "CHAR", "modbus.diagnostic.ascii_input_delimiter",
            FT_UINT8, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_bus_message_count,
            { "Total Message Count", "modbus.diagnostic.bus_message_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_bus_comm_error_count,
            { "CRC Error Count", "modbus.diagnostic.bus_comm_error_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_bus_exception_error_count,
            { "Exception Error Count", "modbus.diagnostic.bus_exception_error_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_slave_message_count,
            { "Slave Message Count", "modbus.diagnostic.slave_message_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_no_slave_response_count,
            { "Slave No Response Count", "modbus.diagnostic.no_slave_response_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_slave_nak_count,
            { "Slave NAK Count", "modbus.diagnostic.slave_nak_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_slave_busy_count,
            { "Slave Device Busy Count", "modbus.diagnostic.slave_busy_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_return_bus_char_overrun_count,
            { "Slave Character Overrun Count", "modbus.diagnostic.bus_char_overrun_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_status,
            { "Status",                        "modbus.ev_status",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_event_count,
            { "Event Vount",                   "modbus.ev_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_message_count,
            { "Message Count",                 "modbus.ev_msg_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_comm_err,
            { "Communication Error",           "modbus.ev_recv_comm_err",
            FT_UINT8, BASE_DEC, NULL, 0x02,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_char_over,
            { "Character Overrun",             "modbus.ev_recv_char_over",
            FT_UINT8, BASE_DEC, NULL, 0x10,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_lo_mode,
            { "Currently in Listen Only Mode", "modbus.ev_recv_lo_mode",
            FT_UINT8, BASE_DEC, NULL, 0x20,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_broadcast,
            { "Broadcast Received",            "modbus.ev_recv_broadcast",
            FT_UINT8, BASE_DEC, NULL, 0x40,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_read_ex,
            { "Read Exception Sent",            "modbus.ev_send_read_ex",
            FT_UINT8, BASE_DEC, NULL, 0x01,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_slave_abort_ex,
            { "Slave Abort Exception Sent",     "modbus.ev_send_slave_abort_ex",
            FT_UINT8, BASE_DEC, NULL, 0x02,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_slave_busy_ex,
            { "Slave Busy Exception Sent",      "modbus.ev_send_slave_busy_ex",
            FT_UINT8, BASE_DEC, NULL, 0x04,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_slave_nak_ex,
            { "Slave Program NAK Exception Sent", "modbus.ev_send_slave_nak_ex",
            FT_UINT8, BASE_DEC, NULL, 0x08,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_write_timeout,
            { "Write Timeout Error Occurred",  "modbus.ev_send_write_timeout",
            FT_UINT8, BASE_DEC, NULL, 0x10,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_lo_mode,
            { "Currently in Listen Only Mode", "modbus.ev_send_lo_mode",
            FT_UINT8, BASE_DEC, NULL, 0x20,
            NULL, HFILL }
        },
        { &hf_modbus_andmask,
            { "AND mask",                      "modbus.and_mask",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_ormask,
            { "OR mask",                       "modbus.or_mask",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_data,
            { "Data",                          "modbus.data",
            FT_BYTES,  BASE_NONE, NULL,    0x0, NULL, HFILL }
        },
        { &hf_modbus_mei,
            { "MEI type",                      "modbus.mei",
            FT_UINT8, BASE_DEC, VALS(encap_interface_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_read_device_id,
            { "Read Device ID",                "modbus.read_device_id",
            FT_UINT8, BASE_DEC, VALS(read_device_id_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_object_id,
            { "Object ID",                     "modbus.object_id",
            FT_UINT8, BASE_DEC, VALS(object_id_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_num_objects,
            { "Number of Objects",             "modbus.num_objects",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_list_object_len,
            { "Object length",                 "modbus.objects_len",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_conformity_level,
            { "Conformity Level",              "modbus.conformity_level",
            FT_UINT8, BASE_HEX, VALS(conformity_level_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_more_follows,
            { "More Follows",                  "modbus.more_follows",
            FT_UINT8, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_next_object_id,
            { "Next Object ID",                "modbus.next_object_id",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_object_str_value,
            { "Object String Value",           "modbus.object_str_value",
            FT_STRING, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reg_uint16,
            { "Register (UINT16)",             "modbus.register.uint16",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reg_uint32,
            { "Register (UINT32)",             "modbus.register.uint32",
            FT_UINT32, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reg_ieee_float,
            { "Register (IEEE Float)",         "modbus.register.ieee_float",
            FT_FLOAT, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reg_modicon_float,
            { "Register (Modicon Float)",      "modbus.register.modicon_float",
            FT_FLOAT, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        },
    };

    /* Setup protocol subtree array */
    static gint *ett[] = {
        &ett_mbtcp,
        &ett_mbrtu,
        &ett_modbus_hdr,
        &ett_group_hdr,
        &ett_events,
        &ett_events_recv,
        &ett_events_send,
        &ett_device_id_objects,
        &ett_device_id_object_items
    };

    static ei_register_info ei[] = {
        { &ei_modbus_data_decode, { "modbus.data.decode", PI_PROTOCOL, PI_WARN, "Invalid decoding options, register data not a multiple of 4!", EXPFILL }},
    };

    module_t *mbtcp_module;
    module_t *mbrtu_module;
    expert_module_t* expert_mbrtu;
    expert_module_t* expert_modbus;

    /* Register the protocol name and description */
    proto_mbtcp = proto_register_protocol("Modbus/TCP", "Modbus/TCP", "mbtcp");
    proto_mbrtu = proto_register_protocol("Modbus RTU", "Modbus RTU", "mbrtu");
    proto_modbus = proto_register_protocol("Modbus", "Modbus", "modbus");

    /* Registering protocol to be called by another dissector */
    modbus_handle = new_register_dissector("modbus", dissect_modbus, proto_modbus);
    mbtcp_handle = new_register_dissector("mbtcp", dissect_mbtcp, proto_mbtcp);
    mbrtu_handle = new_register_dissector("mbrtu", dissect_mbrtu, proto_mbrtu);

    /* Registering subdissectors table */
    modbus_data_dissector_table = register_dissector_table("modbus.data", "Modbus Data", FT_STRING, BASE_NONE);
    modbus_dissector_table = register_dissector_table("mbtcp.prot_id", "Modbus/TCP protocol identifier", FT_UINT16, BASE_DEC);

    /* Required function calls to register the header fields and subtrees used */
    proto_register_field_array(proto_mbtcp, mbtcp_hf, array_length(mbtcp_hf));
    proto_register_field_array(proto_mbrtu, mbrtu_hf, array_length(mbrtu_hf));
    proto_register_field_array(proto_modbus, hf, array_length(hf));
    proto_register_subtree_array(ett, array_length(ett));
    expert_mbrtu = expert_register_protocol(proto_mbrtu);
    expert_register_field_array(expert_mbrtu, mbrtu_ei, array_length(mbrtu_ei));
    expert_modbus = expert_register_protocol(proto_modbus);
    expert_register_field_array(expert_modbus, ei, array_length(ei));


    /* Register required preferences for Modbus Protocol register decoding */
    mbtcp_module = prefs_register_protocol(proto_mbtcp, proto_reg_handoff_mbtcp);
    mbrtu_module = prefs_register_protocol(proto_mbrtu, proto_reg_handoff_mbrtu);

    /* Modbus RTU Preference - Desegment, defaults to TRUE for TCP desegmentation */
    prefs_register_bool_preference(mbtcp_module, "desegment",
                                  "Desegment all Modbus RTU packets spanning multiple TCP segments",
                                  "Whether the Modbus RTU dissector should desegment all messages spanning multiple TCP segments",
                                  &mbtcp_desegment);

    /* Modbus/TCP Preference - Default TCP Port, allows for "user" port either than 502. */
    prefs_register_uint_preference(mbtcp_module, "tcp.port", "Modbus/TCP Port",
                       "Set the TCP port for Modbus/TCP packets (if other"
                       " than the default of 502)",
                       10, &global_mbus_tcp_port);

    /* Modbus/TCP Preference - Holding/Input Register format, this allows for deeper dissection of response data */
    prefs_register_enum_preference(mbtcp_module, "mbus_register_format",
                                    "Holding/Input Register Format",
                                    "Register Format",
                                    &global_mbus_tcp_register_format,
                                    mbus_register_format,
                                    TRUE);

    /* Modbus/TCP Preference - Register addressing format, this allows for a configurable display option to determine addressing used */
    prefs_register_enum_preference(mbtcp_module, "mbus_register_addr_type",
                                    "Register Addressing Type",
                                    "Register Addressing Type (Raw, Modicon 5 or 6). This option has no effect on the underlying protocol, but changes the register address display format",
                                    &global_mbus_tcp_register_addr_type,
                                    mbus_register_addr_type,
                                    TRUE);

    /* Modbus RTU Preference - Desegment, defaults to TRUE for TCP desegmentation */
    prefs_register_bool_preference(mbrtu_module, "desegment",
                                  "Desegment all Modbus RTU packets spanning multiple TCP segments",
                                  "Whether the Modbus RTU dissector should desegment all messages spanning multiple TCP segments",
                                  &mbrtu_desegment);

    /* Modbus RTU Preference - CRC verification, defaults to FALSE (not do verification)*/
    prefs_register_bool_preference(mbrtu_module, "crc_verification",
                                  "Validate CRC",
                                  "Whether to validate the CRC",
                                  &mbrtu_crc);

    /* Modbus RTU Preference - Default TCP Port, defaults to zero, allows custom user port. */
    prefs_register_uint_preference(mbrtu_module, "tcp.port", "Modbus RTU Port",
                       "Set the TCP port for encapsulated Modbus RTU packets",
                       10, &global_mbus_rtu_port);

    /* Modbus RTU Preference - Holding/Input Register format, this allows for deeper dissection of response data */
    prefs_register_enum_preference(mbrtu_module, "mbus_register_format",
                                    "Holding/Input Register Format",
                                    "Register Format",
                                    &global_mbus_rtu_register_format,
                                    mbus_register_format,
                                    TRUE);

    /* Modbus RTU Preference - Register addressing format, this allows for a configurable display option to determine addressing used */
    prefs_register_enum_preference(mbrtu_module, "mbus_register_addr_type",
                                    "Register Addressing Type",
                                    "Register Addressing Type (Raw, Modicon 5 or 6). This option has no effect on the underlying protocol, but changes the register address display format",
                                    &global_mbus_rtu_register_addr_type,
                                    mbus_register_addr_type,
                                    TRUE);

}


/* If this dissector uses sub-dissector registration add a registration routine.
   This format is required because a script is used to find these routines and
   create the code that calls these routines.
 */
void
proto_reg_handoff_mbtcp(void)
{
    static unsigned int mbtcp_port;

    /* Make sure to use Modbus/TCP Preferences field to determine default TCP port */

    if(mbtcp_port != 0 && mbtcp_port != global_mbus_tcp_port){
        dissector_delete_uint("tcp.port", mbtcp_port, mbtcp_handle);
    }

    if(global_mbus_tcp_port != 0 && mbtcp_port != global_mbus_tcp_port) {
        dissector_add_uint("tcp.port", global_mbus_tcp_port, mbtcp_handle);
    }

    mbtcp_port = global_mbus_tcp_port;

    dissector_add_uint("mbtcp.prot_id", MODBUS_PROTOCOL_ID, modbus_handle);

}

void
proto_reg_handoff_mbrtu(void)
{
    static unsigned int mbrtu_port = 0;

    /* Make sure to use Modbus RTU Preferences field to determine default TCP port */

    if(mbrtu_port != 0 && mbrtu_port != global_mbus_rtu_port){
        dissector_delete_uint("tcp.port", mbrtu_port, mbrtu_handle);
    }

    if(global_mbus_rtu_port != 0 && mbrtu_port != global_mbus_rtu_port) {
        dissector_add_uint("tcp.port", global_mbus_rtu_port, mbrtu_handle);
    }

    mbrtu_port = global_mbus_rtu_port;

    dissector_add_uint("mbtcp.prot_id", MODBUS_PROTOCOL_ID, modbus_handle);

}
