Variables start_addr and count were read from query_packet using function mb_ntoh_safe. It looks like some compilers change the pointer alignment if the first byte starts at an odd address. Because mb_ntoh_safe uses pointers slave address and count (number of registers) were not read correctly from the buffer when several modbus slaves were present in network. In this temporary solution pointer aritmetics is replaced by simple 256 multiplication.
/*
* Copyright (c) 2002,2016 Mario de Sousa (msousa@fe.up.pt)
*
* This file is part of the Modbus library for Beremiz and matiec.
*
* This Modbus library is free software: you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation, either version 3 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 Lesser
* General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this Modbus library. If not, see <http://www.gnu.org/licenses/>.
*
* This code is made available on the understanding that it will not be
* used in safety-critical situations without a full and competent review.
*/
#ifndef MODBUS_TCP_PRIVATE_H
#define MODBUS_TCP_PRIVATE_H
#include "mb_util.h"
/* tcp port default configuration... */
#define DEF_SERVICE "502" /* port used by modbus */
#define DEF_PROTOCOL "tcp" /* protocol used by modbus tcp */
#define DEF_TYPE SOCK_STREAM /* Quality of service required of the socket... */
#define DEF_MAX_PENDING_CONNECTION_REQUESTS 5
/* maximum number of pending connection requests
* that have not yet been accept()'ed
*/
#define DEF_CLOSE_ON_SILENCE 1 /* Used only by master nodes.
* Flag indicating whether, by default, the connection
* to the slave device should be closed whenever the
* modbus_tcp_silence_init() function is called.
*
* 0 -> do not close connection
* >0 -> close connection
*
* The spec sugests that connections that will not
* be used for longer than 1 second should be closed.
* Even though we expect most connections to have
* silence intervals much shorted than 1 second, we
* decide to use the default of shuting down the
* connections because it is safer, and most other
* implementations seem to do the same.
* If we do not close we risk using up all the possible
* connections that the slave can simultaneouly handle,
* effectively locking out every other master that
* wishes to communicate with that same slave.
*/
/* Since the receive buffer is also re-used to store the frame header,
* we set it to the larger of the two.
*/
#if TCP_HEADER_LENGTH > MAX_L2_FRAME_LENGTH
#define RECV_BUFFER_SIZE TCP_HEADER_LENGTH
#else
#define RECV_BUFFER_SIZE MAX_L2_FRAME_LENGTH
#endif
#endif /* MODBUS_TCP_PRIVATE_H */