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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 lib-
       modbus 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 requests 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
       service.	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	broad-
       cast number.

       In TCP, the slave number	is only	required 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	incor-
       rect  (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 <modbus_get_slave.md>

							    modbus_set_slave()

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