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modbus_send_raw_request() modbus_send_raw_request() Name modbus_send_raw_request - send a raw request Synopsis int modbus_send_raw_request(modbus_t *ctx, const 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 se- lected 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 re- quest 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_HOLD- ING_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", 502); 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 • modbus_receive_confirmation <modbus_receive_confirmation.md> modbus_send_raw_request()
Name | Synopsis | Description | Return value | Example | See also
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