path: root/epan/dissectors/packet-mbtcp.c

/* 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
 *
 * $Id$
 *
 * 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <epan/packet.h>
#include "packet-tcp.h"
#include "packet-mbtcp.h"

/* Initialize the protocol and registered fields */
static int proto_mbtcp = -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_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;

/* Initialize the subtree pointers */
static gint ett_mbtcp = -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 dissector_table_t   mbtcp_dissector_table;
static dissector_table_t   modbus_dissector_table;
static dissector_handle_t  modbus_handle;

static gboolean mbtcp_desegment = TRUE;

static int
classify_packet(packet_info *pinfo)
{
    /* see if nature of packets can be derived from src/dst ports */
    /* if so, return as found */
    if (( pinfo->srcport == PORT_MBTCP ) && ( pinfo->destport != PORT_MBTCP ))
        return RESPONSE_PACKET;
    if (( pinfo->srcport != PORT_MBTCP ) && ( pinfo->destport == PORT_MBTCP ))
        return QUERY_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_INPUT_DISCRETES,   "Read input discretes" },
    { READ_MULT_REGS,         "Read multiple registers" },
    { READ_INPUT_REGS,        "Read input registers" },
    { WRITE_COIL,             "Write 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" },
    { ILLEGAL_RESPONSE,    "Illegal response length" },
    { 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 }
};

/* 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             }
};

/* Code to actually dissect the packets */
static void
dissect_mbtcp_pdu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
/* 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;

    /* 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(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(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_packet(pinfo);

    if (check_col(pinfo->cinfo, COL_INFO))
    {
        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);
            }
        }
    }

    /* if a tree exists, perform operations to add fields to it */
    if (tree) 
    {
        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 packet context */
        p_save_proto_data = p_get_proto_data( pinfo->fd, proto_mbtcp );
        p_remove_proto_data(pinfo->fd, proto_mbtcp);
        p_add_proto_data(pinfo->fd, proto_mbtcp, GINT_TO_POINTER(packet_type));

        /* Show the undissected payload */
        if( tvb_length_remaining(tvb, offset) > 0 )
            call_dissector(modbus_handle, next_tvb, pinfo, tree);

        p_remove_proto_data(pinfo->fd, proto_mbtcp);
        p_add_proto_data(pinfo->fd, proto_mbtcp, p_save_proto_data);
    }
}

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;
}

static int
dissect_mbtcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    /* Make sure there's at least enough data to determine its a Modbus 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);

    return tvb_length(tvb);
}

static int
dissect_mbudp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    /* Make sure there's at least enough data to determine its a Modbus packet */
    if (!tvb_bytes_exist(tvb, 0, 8))
        return 0;

    /* check that it actually looks like Modbus/UDP */
    /* 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;
    }

    /* dissect the PDU */
    dissect_mbtcp_pdu(tvb, pinfo, tree);

    return tvb_length(tvb);
}

/* Code to allow special handling of mbtcp data */
static void
dissect_mbtcp_data(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, guint8 function_code, gint payload_start, gint payload_len)
{
    gint reported_len;
    tvbuff_t *next_tvb;

    reported_len = tvb_reported_length_remaining(tvb, payload_start);

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

	if ( payload_len != reported_len ) {
        proto_tree_add_bytes_format(tree, hf_modbus_data, tvb, payload_start, payload_len, NULL, "Data");
        return;
    }

    next_tvb = tvb_new_subset(tvb, payload_start, payload_len, reported_len);

    switch ( function_code ) {
        default:
            if ( ! dissector_try_string(mbtcp_dissector_table, "data", next_tvb, pinfo, tree) )
                proto_tree_add_bytes_format(tree, hf_modbus_data, tvb, payload_start, payload_len, NULL, "Data");
            break;
    }
}

/* Code to actually dissect the packets */
static int
dissect_modbus(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    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, group_offset, packet_type, temp_data;
    const char    *func_string = "";
    gint          payload_start, payload_len, event_index,
                  i, 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;
    guint16       diagnostic_code;
    guint8        *object_str;

    /* Don't need to do anything if there's no tree */
    if (tree == NULL)
        return tvb_length(tvb);

    len = tvb_length_remaining(tvb, 0);
    function_code = tvb_get_guint8(tvb, 0) & 0x7F;

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

    /* "Request" or "Response" */
    packet_type = GPOINTER_TO_INT(p_get_proto_data( pinfo->fd, proto_mbtcp ));

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

    /* 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);

    /** detail payload as a function of exception/function code */
    func_string = val_to_str(function_code, function_code_vals, "Unknown function");

    payload_start = offset + 1;
    payload_len = len - 1;
    if (exception_code != 0) {
        proto_item_set_text(mi, "function %u:  %s.  Exception: %s",
                            function_code,
                            func_string,
                            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 {
        proto_item_set_text(mi, "function %u:  %s", function_code,
                            func_string);
        switch (function_code) {

            case READ_COILS:
            case READ_INPUT_DISCRETES:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, FALSE);
                }
                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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt);
                }
                break;

            case READ_MULT_REGS:
            case READ_INPUT_REGS:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, FALSE);
                }
                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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt);
                }
                break;

            case WRITE_COIL:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 1);
                    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, FALSE);
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 1);
                    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, FALSE);
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 2);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 2, 2);
                }
                break;

            case READ_EXCEPT_STAT:
                if (packet_type == RESPONSE_PACKET)
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start, 1);
                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_text(modbus_tree, tvb, payload_start+2, payload_len-2, "Request Data");
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                if (payload_len > 2)
                                    proto_tree_add_text(modbus_tree, tvb, payload_start+2, payload_len-2, "Echo Data");
                  	    }
                            break;
                        case RESTART_COMMUNICATION_OPTION:
                            temp_data = tvb_get_ntohs(tvb, payload_start+2);
                            if (temp_data == 0) {
                                proto_tree_add_text(modbus_tree, tvb, payload_start+2, 2, "Leave Log");
                  	    }
                            else if (temp_data == 0xFF) {
                                proto_tree_add_text(modbus_tree, tvb, payload_start+2, 2, "Clear Log");
                  	    }
                            else {
                                proto_tree_add_text(modbus_tree, tvb, payload_start+2, 2, "Unknown");
                  	    }
                            break;
                        case RETURN_DIAGNOSTIC_REGISTER:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Diagnostic Register Contents 0x%04x", temp_data );
                  	    }
                            break;
                        case CHANGE_ASCII_INPUT_DELIMITER:
                            temp_data = tvb_get_guint8(tvb, payload_start+2);
                            proto_tree_add_text( modbus_tree, tvb, payload_start+2, 1, "CHAR 0x%02x", temp_data );
                            break;
                        case RETURN_BUS_MESSAGE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Total Message Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_BUS_COMM_ERROR_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "CRC Error Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_BUS_EXCEPTION_ERROR_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Exception Error Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_SLAVE_MESSAGE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Slave Message Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_SLAVE_NO_RESPONSE_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Slave No Response Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_SLAVE_NAK_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Slave NAK Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_SLAVE_BUSY_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Slave Device Busy Count %d", temp_data );
                  	    }
                            break;
                        case RETURN_BUS_CHAR_OVERRUN_COUNT:
                            if (packet_type == QUERY_PACKET) {
                                if (payload_len > 2)
                                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                  	    }
                            else if (packet_type == RESPONSE_PACKET) {
                                temp_data = tvb_get_ntohs(tvb, payload_start+2);
                                proto_tree_add_text( modbus_tree, tvb, payload_start+2, 2, "Slave Character Overrun Count %d", temp_data );
                  	    }
                            break;
                        case CLEAR_OVERRUN_COUNTER_AND_FLAG:
                        case FORCE_LISTEN_ONLY_MODE:
                        case CLEAR_COUNTERS_AND_DIAG_REG:
                        default:
                            if (payload_len > 2)
                                dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start+2, payload_len-2);
                            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, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_event_count, tvb, payload_start+2, 2, FALSE);
                }
                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, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_event_count, tvb, payload_start+3, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_message_count, tvb, payload_start+5, 2, FALSE);
                    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, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_char_over,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_lo_mode,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_recv_broadcast,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                            }
                            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, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_abort_ex,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_busy_ex,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_slave_nak_ex,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_write_timeout,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                                proto_tree_add_item(event_item_tree, hf_modbus_event_send_lo_mode,
                                  tvb, payload_start+7+event_index, 1, TRUE );
                            }
                            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, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, FALSE);
                    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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 5, byte_cnt);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_bitcnt, tvb, payload_start + 2, 2, FALSE);
                }
                break;

            case WRITE_MULT_REGS:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, FALSE);
                    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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 5, byte_cnt);
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_wordcnt, tvb, payload_start + 2, 2, FALSE);
                }
                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 (i = 0; i < byte_cnt / 7; i++) {
                        mi = proto_tree_add_text( modbus_tree, tvb, group_offset, 7,
                                "Group %u", i);
                        group_tree = proto_item_add_subtree(mi, ett_group_hdr);
                        proto_tree_add_item(group_tree, hf_modbus_reftype, tvb, group_offset, 1, FALSE);
                        proto_tree_add_item(group_tree, hf_modbus_lreference, tvb, group_offset + 1, 4, FALSE);
                        proto_tree_add_item(group_tree, hf_modbus_wordcnt, tvb, group_offset + 5, 2, FALSE);
                        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;
                    i = 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", i);
                        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, FALSE);
                        dissect_mbtcp_data(tvb, pinfo, group_tree, function_code, group_offset + 2, group_byte_cnt - 1);
                        group_offset += (group_byte_cnt + 1);
                        byte_cnt -= (group_byte_cnt + 1);
                        i++;
                    }
                }
                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;
                    i = 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", i);
                        group_tree = proto_item_add_subtree(mi, ett_group_hdr);
                        proto_tree_add_item(group_tree, hf_modbus_reftype, tvb, group_offset, 1, FALSE);
                        proto_tree_add_item(group_tree, hf_modbus_lreference, tvb, group_offset + 1, 4, FALSE);
                        proto_tree_add_uint(group_tree, hf_modbus_wordcnt, tvb, group_offset + 5, 2,
                                group_word_cnt);
                        dissect_mbtcp_data(tvb, pinfo, group_tree, function_code, group_offset + 7, group_byte_cnt - 7);
                        group_offset += group_byte_cnt;
                        byte_cnt -= group_byte_cnt;
                        i++;
                    }
                }
                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, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_andmask, tvb, payload_start + 2, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_ormask, tvb, payload_start + 4, 2, FALSE);
                }
                break;

            case READ_WRITE_REG:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_readref, tvb, payload_start, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_readwordcnt, tvb, payload_start + 2, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_writeref, tvb, payload_start + 4, 2, FALSE);
                    proto_tree_add_item(modbus_tree, hf_modbus_writewordcnt, tvb, payload_start + 6, 2, FALSE);
                    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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 9, byte_cnt);
                }
                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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 1, byte_cnt);
                }
                break;

            case READ_FIFO_QUEUE:
                if (packet_type == QUERY_PACKET)
                    proto_tree_add_item(modbus_tree, hf_modbus_reference, tvb, payload_start, 2, FALSE);
                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, FALSE);
                    dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start + 4, byte_cnt - 2);
                }
                break;

            case ENCAP_INTERFACE_TRANSP:
                if (packet_type == QUERY_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_mei, tvb, payload_start, 1, FALSE);
                    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, FALSE);
                            proto_tree_add_item(modbus_tree, hf_modbus_object_id, tvb, payload_start+2, 1, FALSE);
                            break;

                        case CANOPEN_REQ_RESP:
                            /* CANopen protocol not part of the Modbus/TCP specification */
                        default:
                            if (payload_len > 1)
                                dissect_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start, payload_len-1);
                                break;
                    }
                }
                else if (packet_type == RESPONSE_PACKET) {
                    proto_tree_add_item(modbus_tree, hf_modbus_mei, tvb, payload_start, 1, FALSE);
                    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, FALSE);
                            proto_tree_add_item(modbus_tree, hf_modbus_conformity_level, tvb, payload_start+2, 1, FALSE);
                            proto_tree_add_item(modbus_tree, hf_modbus_more_follows, tvb, payload_start+3, 1, FALSE);
                            proto_tree_add_item(modbus_tree, hf_modbus_next_object_id, tvb, payload_start+4, 1, FALSE);
                            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 (i = 1; i <= num_objects; i++)
                            {
                                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", i);
                                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, FALSE);
                                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)
                                {
                                    object_str = tvb_get_string(tvb, payload_start+6+object_index, object_len);
                                    proto_tree_add_string(device_objects_item_tree, hf_modbus_object_str_value, tvb, payload_start+6+object_index, object_len, object_str);
                                    g_free(object_str);
                     		}
                                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_mbtcp_data(tvb, pinfo, modbus_tree, function_code, payload_start, payload_len-1);
                                break;
                    }
                }
                break;

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

    return tvb_length(tvb);
}


/* Register the protocol with Wireshark */

void
proto_register_modbus(void)
{

/* Setup list of header fields  See Section 1.6.1 for details*/
    static hf_register_info hf[] = {
        /* Modbus/TCP header fields */
        { &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 }
        },
        /* Modbus header fields */
        { &hf_mbtcp_unitid,
            { "unit identifier",                "mbtcp.modbus.unit_id",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_mbtcp_functioncode,
            { "function code",                "mbtcp.modbus.func_code",
            FT_UINT8, BASE_DEC, VALS(function_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reference,
            { "reference number",                "mbtcp.modbus.reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_lreference,
            { "reference number (32 bit)",            "mbtcp.modbus.reference_num_32",
            FT_UINT32, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_reftype,
            { "reference type",                "mbtcp.modbus.reference_type",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_readref,
            { "read reference number",            "mbtcp.modbus.read_reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_writeref,
            { "write reference number",            "mbtcp.modbus.write_reference_num",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_wordcnt,
            { "word count",                    "mbtcp.modbus.word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_readwordcnt,
            { "read word count",                "mbtcp.modbus.read_word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_writewordcnt,
            { "write word count",                "mbtcp.modbus.write_word_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_bitcnt,
            { "bit count",                    "mbtcp.modbus.bit_cnt",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_bytecnt,
            { "byte count",                    "mbtcp.modbus.byte_cnt",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_lbytecnt,
            { "byte count (16-bit)",            "mbtcp.modbus.byte_cnt_16",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_exceptioncode,
            { "exception code",                "mbtcp.modbus.exception_code",
            FT_UINT8, BASE_DEC, VALS(exception_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_diag_sf,
            { "diagnostic code",                    "mbtcp.modbus.diagnostic_code",
            FT_UINT16, BASE_DEC, VALS(diagnostic_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_status,
            { "status",                             "mbtcp.modbus.ev_status",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_event_count,
            { "event count",                    "mbtcp.modbus.ev_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_message_count,
            { "message count",               "mbtcp.modbus.ev_msg_count",
            FT_UINT16, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_comm_err,
            { "Communication Error",               "mbtcp.modbus.ev_recv_comm_err",
            FT_UINT8, BASE_DEC, NULL, 0x02,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_char_over,
            { "Character Overrun",               "mbtcp.modbus.ev_recv_char_over",
            FT_UINT8, BASE_DEC, NULL, 0x10,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_lo_mode,
            { "Currently in Listen Only Mode",   "mbtcp.modbus.ev_recv_lo_mode",
            FT_UINT8, BASE_DEC, NULL, 0x20,
            NULL, HFILL }
        },
        { &hf_modbus_event_recv_broadcast,
            { "Broadcast Received",               "mbtcp.modbus.ev_recv_broadcast",
            FT_UINT8, BASE_DEC, NULL, 0x40,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_read_ex,
            { "Read Exception Sent",               "mbtcp.modbus.ev_send_read_ex",
            FT_UINT8, BASE_DEC, NULL, 0x01,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_slave_abort_ex,
            { "Slave Abort Exception Sent",               "mbtcp.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",               "mbtcp.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",               "mbtcp.modbus.ev_send_slave_nak_ex",
            FT_UINT8, BASE_DEC, NULL, 0x08,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_write_timeout,
            { "Write Timeout Error Occurred",               "mbtcp.modbus.ev_send_write_timeout",
            FT_UINT8, BASE_DEC, NULL, 0x10,
            NULL, HFILL }
        },
        { &hf_modbus_event_send_lo_mode,
            { "Currently in Listen Only Mode",               "mbtcp.modbus.ev_send_lo_mode",
            FT_UINT8, BASE_DEC, NULL, 0x20,
            NULL, HFILL }
        },
        { &hf_modbus_andmask,
            { "AND mask",                    "mbtcp.modbus.and_mask",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_ormask,
            { "OR mask",                    "mbtcp.modbus.or_mask",
            FT_UINT16, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_data,
            { "Data",                        "mbtcp.modbus.data",
            FT_BYTES,  BASE_NONE, NULL,    0x0, NULL, HFILL }
        },
        { &hf_modbus_mei,
            { "MEI type",                        "mbtcp.modbus.mei",
            FT_UINT8, BASE_DEC, VALS(encap_interface_code_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_read_device_id,
            { "Read Device ID",                "mbtcp.modbus.read_device_id",
            FT_UINT8, BASE_DEC, VALS(read_device_id_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_object_id,
            { "Object ID",                        "mbtcp.modbus.object_id",
            FT_UINT8, BASE_DEC, VALS(object_id_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_num_objects,
            { "Number of Objects",                        "mbtcp.modbus.num_objects",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_list_object_len,
            { "Object length",                        "mbtcp.modbus.objects_len",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_conformity_level,
            { "Conformity Level",                        "mbtcp.modbus.conformity_level",
            FT_UINT8, BASE_HEX, VALS(conformity_level_vals), 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_more_follows,
            { "More Follows",                        "mbtcp.modbus.more_follows",
            FT_UINT8, BASE_HEX, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_next_object_id,
            { "Next Object ID",                        "mbtcp.modbus.next_object_id",
            FT_UINT8, BASE_DEC, NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_modbus_object_str_value,
            { "Object String Value",                        "mbtcp.modbus.object_str_value",
            FT_STRING, BASE_NONE, NULL, 0x0,
            NULL, HFILL }
        }
    };

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

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

    /* Registering protocol to be called by another dissector */
    new_register_dissector("mbtcp", dissect_mbtcp, proto_mbtcp);
    new_register_dissector("modbus", dissect_modbus, proto_modbus);

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

    /* Required function calls to register the header fields and subtrees used */
    proto_register_field_array(proto_mbtcp, hf, array_length(hf));
    proto_register_subtree_array(ett, array_length(ett));
}


/* 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)
{
    dissector_handle_t mbtcp_handle, mbudp_handle;

    mbtcp_handle = new_create_dissector_handle(dissect_mbtcp, proto_mbtcp);
    dissector_add_uint("tcp.port", PORT_MBTCP, mbtcp_handle);

    mbudp_handle = new_create_dissector_handle(dissect_mbudp, proto_mbtcp);
    dissector_add_uint("udp.port", PORT_MBTCP, mbudp_handle);

    modbus_handle = new_create_dissector_handle(dissect_modbus, proto_modbus);
    dissector_add_uint("mbtcp.prot_id", MODBUS_PROTOCOL_ID, modbus_handle);
}