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.
The public header of libmodbus provides a list of supported Modbus functions
codes, prefixed by MODBUS_FC_
(eg. MODBUS_FC_READ_HOLDING_REGISTERS
), to help
build of raw requests.
RETURN VALUE
The 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
modbus_t *ctx; /* Read 5 holding registers from address 1 */ uint8_t raw_req[] = { 0xFF, MODBUS_FC_READ_HOLDING_REGISTERS, 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_confirmation(ctx, rsp); modbus_close(ctx); modbus_free(ctx);
SEE ALSO
AUTHORS
The libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com>