modbus_set_slave.txt

modbus_set_slave(3)
===================


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 holing 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. 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
-------
[source,c]
-------------------
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;
}
-------------------

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