ModbusInvalidResponseError: Response length is invalid 0

[StevenLau6]

I am trying to use a Raspberry Pi 3B as a Master to read values from an electric energy meter which supports Modbus-RTU protocol. I used a RS232/USB adapter and a RS485/RS232 adapter to link the meter and the USB port on the Raspberry Pi. When I run the following code：

import sys
import serial

#add logging capability
import logging
import modbus_tk
import modbus_tk.defines as cst
import modbus_tk.modbus_rtu as modbus_rtu

logger = modbus_tk.utils.create_logger("console")
if __name__ == "__main__":
    try:
        #Connect to the slave
        master = modbus_rtu.RtuMaster(serial.Serial(port="/dev/ttyUSB0", baudrate=9600, bytesize=8, parity='N', stopbits=1, xonxoff=0))
        master.set_timeout(5.0)    #Change the timeout value/Defines a timeout on the MAC layer
        master.set_verbose(True)   #print some more log prints for debug purpose
        logger.info("connected")

        logger.info(master.execute(1, cst.READ_HOLDING_REGISTERS, 0, 49))

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

The port, baudrate, bytesize,parity,and stopbits were set correctly, but it always returns this:

2017-08-10 19:24:34,282 INFO    modbus_rtu.__init__ MainThread  RtuMaster /dev/ttyUSB0 is opened
2017-08-10 19:24:34,283 INFO    rtumaster_example.<module>  MainThread  connected
2017-08-10 19:24:34,284 DEBUG   modbus.execute  MainThread  -> 1-3-0-0-0-49-132-30
2017-08-10 19:24:39,291 DEBUG   modbus.execute  MainThread  <-
Traceback (most recent call last):
  File "rtumaster_example.py", line 34, in <module>
    logger.info(master.execute(1, cst.READ_HOLDING_REGISTERS, 0, 49))
  File "build/bdist.linux-x86_64/egg/modbus_tk/utils.py", line 39, in new
modbus_tk.exceptions.ModbusInvalidResponseError: Response length is invalid 0

And then I use the same serial transmission line to link the meter and the laptop, and use a debugging tool running on Windows XP, which was developed by the manufacturer of the meter. The debugging tool sends the same Request (1-3-0-0-0-49-132-30) as before, but the debugging tool can get correct Responses. (Maybe it's because it ignored some incorrect Responses and keep on send Requests regularly) I have tried many times, but the Raspberry Pi never gets a Response, and always returns the same error information.

I am new to Modbus and serial communication, so any help would be appreciated.

[StevenLau6]

I have solved my problem by using a more expensive USBtoRS485 connector. The problem took me a lot of time to try different library and different code. It turns out that the adapter "QinHeng Electronics HL-340 usb-serial" that I bought works well on windows but does not work with Linux. It can achieve send and receive message on Linux, but it just can`t support Modbus communication.

So I suggest you buy a more expensive connector, and it may save your a lot of time and energy.

That`s all, thank you!

[RyanChen9527]

On the contrary, it worked perfectly with QinHeng Electronics HL-340 for me. i will upload my project later.

[weiyigit]

Please try code.

self.inst = modbus_rtu.RtuMaster(serial.Serial(port=com, baudrate=9600, bytesize=8, parity='N', stopbits=1, xonxoff=0))
self.inst.set_timeout(timeout)
self.inst.set_verbose(True)