Error communicating with Arduino uno using modbus-tk: Response length is invalid 0

[anshulpaigwar]

I am trying to communicate with Arduino Uno (slave) using modbus-tk from my PC (master) with following code:

import serial

import modbus_tk
import modbus_tk.defines as cst
from modbus_tk import modbus_rtu

PORT = "COM6"
#PORT = '/dev/ttyp5'

def main():
    """main"""
    logger = modbus_tk.utils.create_logger("console")

    try:
        #Connect to the slave
        master = modbus_rtu.RtuMaster(
            serial.Serial(port=PORT, baudrate=19200, bytesize=8, parity='N', stopbits=1, xonxoff=0)
        )
        master.set_timeout(0.5)
        master.set_verbose(True)
        logger.info("connected")

        logger.info(master.execute(1, cst.READ_HOLDING_REGISTERS,0,16))

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

if __name__ == "__main__":
    main()

I am getting following error: modbus_tk.exceptions.ModbusInvalidResponseError: Response length is invalid 0

2017-02-07 12:37:51,493 INFO    modbus_rtu.__init__ MainThread  RtuMaster COM6 is opened
2017-02-07 12:37:51,502 INFO    rtumaster_example.main  MainThread  connected
2017-02-07 12:37:51,523 DEBUG   modbus.execute  MainThread  -> 1-3-0-0-0-16-68-6
2017-02-07 12:37:52,025 DEBUG   modbus.execute  MainThread  <-
Traceback (most recent call last):
  File "D:\ANSHUL\MakeServo\MakeServo code\modbus-tk-master\examples\rtumaster_example.py", line 50, in <module>
    main()
  File "D:\ANSHUL\MakeServo\MakeServo code\modbus-tk-master\examples\rtumaster_example.py", line 34, in main
    logger.info(master.execute(1, cst.READ_HOLDING_REGISTERS,0,16))
  File "build\bdist.win-amd64\egg\modbus_tk\utils.py", line 42, in new
modbus_tk.exceptions.ModbusInvalidResponseError: Response length is invalid 0
[Finished in 0.77s]

I have tested my Arduino code with Qmodbus and it is working absolutely fine but and getting proper response. But I am unable to solve the error with modbus-tk. Hope someone could help me. thanks in advance.

[ljean]

This look like a communication error. Did you check that the serial port is correctly opened and that data is sent to the slave?

[g0lln3r]

I had the same problem, what helped was to add a line after connecting to the arduino where you wait two seconds, because the arduino resets after opening the port. This should work. You can also put a capacitor between the reset pin and ground on your arduino.

`import serial

import time import modbus_tk import modbus_tk.defines as cst from modbus_tk import modbus_rtu

PORT = "COM6"

PORT = '/dev/ttyp5'

def main(): """main""" logger = modbus_tk.utils.create_logger("console")

try:
    #Connect to the slave
    master = modbus_rtu.RtuMaster(
        serial.Serial(port=PORT, baudrate=19200, bytesize=8, parity='N', stopbits=1, xonxoff=0)
    )
    master.set_timeout(0.5)
    master.set_verbose(True)
    logger.info("connected")

    time.sleep(2)

    logger.info(master.execute(1, cst.READ_HOLDING_REGISTERS,0,16))

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

if name == "main": main()`

[johngai19]

@g0lln3r I encountered the same problem and you way solved it, thank you very much for share

[FluffyPancakeLove]

i have encountered the same error but unfortunately this did not help me