Hi used function modbus_rtu_set_custom_rts(), for manage trasmition direction using a gpio of my HW. I insert this function in main modbus ... ctxRTU = modbus_new_rtu("/dev/ttyS0", 19200, 'N', 8, 1); modbus_connect(ctxRTU); modbus_rtu_set_custom_rts(ctxRTU, csetrts); .....