Modbus TCP error

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  • June 30, 2015 at 11:58 am #243
    mitchok
    Participant

    Hi

    I am trying to use Rapid SCADA to communicate with a set up Northern Design Modbus over TCP meters. It seems that communications is established but an error is being generated. Looking at the log in communicator shows ‘Incorrect PDU length’

    Can someone point me in the right direction as to how I can deal with this please?

    Thanks
    Darren

    June 30, 2015 at 12:02 pm #244
    Mikhail
    Moderator

    Hello Darren,
    Could you write a piece of log here?

    June 30, 2015 at 12:14 pm #245
    mitchok
    Participant

    See below

    Thanks
    Darren

    2015-06-30 12:57:47 Communication session with Device 50 “CH#1”, type: KpModbus, call number: 192.168.5.19
    Request element group
    Send (12): 00 00 00 00 00 06 00 04 0A FF 00 01
    Receive (7/7): 00 00 00 00 00 06 00
    Receive (5/5): 04 0A FF 00 01
    Incorrect PDU length!
    Request element group
    Send (12): 00 00 00 00 00 06 00 04 0A FF 00 01
    Receive (7/7): 00 00 00 00 00 06 00
    Receive (5/5): 04 0A FF 00 01
    Incorrect PDU length!
    Request element group
    Send (12): 00 00 00 00 00 06 00 04 0A FF 00 01
    Receive (7/7): 00 00 00 00 00 06 00
    Receive (5/5): 04 0A FF 00 01
    Incorrect PDU length!

    June 30, 2015 at 3:25 pm #246
    Mikhail
    Moderator

    The device responses the same bytes as were requested.
    According to MODBUS APPLICATION PROTOCOL SPECIFICATION and MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE the content of the response is definitely wrong.

    You can get proper communication log for comparison by using Modbus emulator mod_RSsim

    July 1, 2015 at 8:31 am #247
    mitchok
    Participant

    Thanks for the comments. I have tried communicating with a different unit and have had no trouble so I think I am either using the wrong address or the wrong data type

    In my mind I have a vague memory that in the event of the slave not understanding the request or being unable to comply, it will repeat the request message back! Did I make this up or does this have some element of truth in it?

    As you may guess, I am pretty new to this and trying to implement a metering solution on a very tight budget!

    July 1, 2015 at 5:30 pm #248
    Mikhail
    Moderator

    Probably your vague memorials is about other protocol than Modbus )
    Modbus has special error codes for such cases or device does not respond anything.
    Good luck in experiments! Your questions help others to solve their issues.

    July 3, 2015 at 2:00 pm #249
    mitchok
    Participant

    Thanks for the help. Have managed to get that part working now. It seems I was trying to access a non-existant register !!!!!

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