libmodbus/doc/modbus_send_raw_request.txt

modbus_send_raw_request(3)
==========================


NAME
----
modbus_send_raw_request - send a raw request


SYNOPSIS
--------
*int modbus_send_raw_request(*modbus_t 'ctx', uint8_t *'raw_req, int 'raw_req_length');*


DESCRIPTION
-----------
The _modbus_send_raw_request()_ function shall send a request via the socket of
the context 'ctx'. This function must be used for debugging purposes because you
have to take care to make a valid request by hand. The function only adds to the
message, the header or CRC of the selected backend, so 'raw_req' must start and
contain at least a slave/unit identifier and a function code. This function can
be used to send request not handled by the library.


RETURN VALUE
------------
The _modbus_send_raw_request()_ function shall return the full message length,
counting the extra data relating to the backend, if successful. Otherwise it
shall return -1 and set errno.


EXAMPLE
-------
[source,c]
-------------------
modbus_t *ctx;
/* Read 5 holding registers from address 1 */
uint8_t raw_req[] = { 0xFF, 0x03, 0x00, 0x01, 0x0, 0x05 };
int req_length;
uint8_t rsp[MODBUS_TCP_MAX_ADU_LENGTH];

ctx = modbus_new_tcp("127.0.0.1", 1502);
if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}

req_length = modbus_send_raw_request(ctx, raw_req, 6 * sizeof(uint8_t));
modbus_receive(ctx, -1, rsp);

modbus_close(ctx);
modbus_free(ctx);
-------------------

SEE ALSO
--------
linkmb:modbus_receive[3]


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