I am trying to use modbus with RS485 on my Pi4. I'm using modbus_tk with pySerial library, and I managed to get controllig RTS pin while transmitting data, but I cant get it running faster - when TX ends the pins stays UP for aprox. 30ms (25 +- 35) after TX.


My code below:

socket = serial.rs485.RS485(port=PORT, baudrate=460800, bytesize=8, parity='N', stopbits=1, rtscts=True, dsrdtr=False, timeout=0.01, xonxoff=0 )
socket.rs485_mode = serial.rs485.RS485Settings(rts_level_for_tx=True, rts_level_for_rx=False, loopback=False, delay_before_tx=0, delay_before_rx=0)

master = modbus_rtu.RtuMaster(socket)

def readAll():
        data = master.execute(1, cst.READ_HOLDING_REGISTERS, 999, 55)
        print("IsolInputs: ", data[0:4])
        print("ADCInputs: ", data[4:21])
        print("ADCInputs_raw: ", data[21:37])
        print("RelayOtuputs: ", data[37:43])
        print("PWMFrequency: ", data[43])
        print("PWMDutyCycle: ", data[44:54])
        print("DevStatus: ", data[54])

    except modbus_tk.modbus.ModbusError as exc:
        logger.error("%s- Code=%d", exc, exc.get_exception_code())

I tried to manually control another GPIO Pin via hooks in modbus_tk lib, but it gives me the simmilar result:

def on_before_write(args):
    GPIO.output(EN_485, GPIO.HIGH)
hooks.install_hook("modbus.Master.before_send", on_before_write)

def on_after_write(args):
    GPIO.output(EN_485,  GPIO.LOW)
hooks.install_hook("modbus.Master.after_send", on_after_write)

Any ideas?


  • Can you explain why this is a problem? RTS means the host (in this case: the Pi) is ready to send data. In theory, this could stay high forever.
    – PMF
    Apr 15, 2020 at 7:14
  • @PMF i Wanat i to control rs485 hałf duplex, and if it’s high for too long my slaves need to wait with response - this makes my program running too long.
    – SURViR
    Apr 15, 2020 at 7:52
  • I see... So for your case the timeout for resetting RTS is to long. One possibility would be to use an NE555 and manually derive a (short) timeout from the data line.
    – PMF
    Apr 15, 2020 at 7:58
  • I noticed that if i manyally send only one message it works perfectly, but when I do it in loop it makes problem like that.
    – SURViR
    Apr 15, 2020 at 9:36
  • Would adding ser.setRTS(False) as soon as possible in your script help?
    – user115418
    Apr 15, 2020 at 21:09


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