NAME

       modbus_set_slave - set slave number in the context

SYNOPSIS

       int modbus_set_slave(modbus_t *ctx, int slave);

DESCRIPTION

       The modbus_set_slave() function shall set the slave number in the libmodbus context.

       The behavior depends of network and the role of the device:

       RTU
           Define the slave ID of the remote device to talk in master mode or set the internal
           slave ID in slave mode. According to the protocol, a Modbus device must only accept
           message holding its slave number or the special broadcast number.

       TCP
           The slave number is only required in TCP if the message must reach a device on a
           serial network. Some not compliant devices or software (such as modpoll) uses the
           slave ID as unit identifier, that’s incorrect (cf page 23 of Modbus Messaging
           Implementation Guide v1.0b) but without the slave value, the faulty remote device or
           software drops the requests! The special value MODBUS_TCP_SLAVE (0xFF) can be used in
           TCP mode to restore the default value.

       The broadcast address is MODBUS_BROADCAST_ADDRESS. This special value must be use when you
       want all Modbus devices of the network receive the request.

RETURN VALUE

       The function shall return 0 if successful. Otherwise it shall return -1 and set errno to
       one of the values defined below.

ERRORS

       EINVAL
           The slave number is invalid.

EXAMPLE

           modbus_t *ctx;

           ctx = modbus_new_rtu("/dev/ttyUSB0", 115200, 'N', 8, 1);
           if (ctx == NULL) {
               fprintf(stderr, "Unable to create the libmodbus context\n");
               return -1;
           }

           rc = modbus_set_slave(ctx, YOUR_DEVICE_ID);
           if (rc == -1) {
               fprintf(stderr, "Invalid slave ID\n");
               modbus_free(ctx);
               return -1;
           }

           if (modbus_connect(ctx) == -1) {
               fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
               modbus_free(ctx);
               return -1;
           }

SEE ALSO

       modbus_get_slave(3)

AUTHORS

       The libmodbus documentation was written by Stéphane Raimbault
       <stephane.raimbault@gmail.com>