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modbus_set_slave()					      modbus_set_slave()

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.

     It is usually only required to set the slave ID in RTU. The meaning of this
     ID  will  be  different  if  your program acts as client (master) or server
     (slave).

     As RTU client, modbus_set_slave() sets the ID of the remote device you want
     to communicate. Be sure to set the slave ID before issuing any  Modbus  re-
     quests on the serial bus. If you communicate with several servers (slaves),
     you can set the slave ID of the remote device before each request.

     As  RTU  server, the slave ID allows the various clients to reach your ser-
     vice. You should use a free ID, once set, this ID should be  known  by  the
     clients  of  the  network.  According to the protocol, a Modbus device must
     only accept message holding its slave number or the special broadcast  num-
     ber.

     In  TCP,  the slave number is only required if the message must reach a de-
     vice 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  de-
     fault 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 <modbus_get_slave.md>

							      modbus_set_slave()

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