Is it possible to ignore the exceptions and keep on send Requests regularly, when I use Modbus-tk?

[zjuso...@gmail.com]

I use the Raspberry Pi 3B(run Ubuntu Mate 16.04 operating system) with a USBtoRS232 connector and a RS232toRS485 converter to fetch realtime data on a electric energy meter which supports Modbus-RTU protocol.

I have tried to use luc's example script as below:

    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()))

But the Raspberry Pi never gets a response from the meter.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

I am sure that the parameters such as port, baudrate, bytesize,parity,and stopbits were set correctly, and the serial transmission line runs fine with a Windows laptop using a Windows based debugging tool, which was developed by the manufacturer of the meter.

The debugging tool send the same Request (1-3-0-0-0-49-132-30)as  before, but the debugging tool can get correct Respondes. (Maybe it's because it ignored some incorrect Respondes and keep on send Requests regularly) And it represents that the Request message is correct and the connection of serial transmission has no problem.

I hope to know if it is possible to ignore the exceptions and keep on send Requests regularly.

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

[Luc Jean]

Hello,

You may try to set a greater timeout and set the `expected_length` optional parameter of the `execute` method

master.execute(1, cst.READ_HOLDING_REGISTERS, 0, 49, expected_length=100)

You may avoid these incorrect responses and keep things more stable.

The RTU master tries to reset buffers before sending a new requests and avoid side effect from the previous request.

I hope it helps

Best

luc

  

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