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. 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 ------- [source,c] ------------------- 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 -------- linkmb:modbus_receive_confirmation[3] AUTHORS ------- The libmodbus documentation was written by Stéphane Raimbault