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    M340 specific system words, %SW138 %SW141

    Goals and Symptoms

    To facilitate the migration from LL984 to M340, four new System Words have been added to offset the starting address of variables which are accessed by Modbus requests.

    Facts and Changes

     

    System Word Modbus request impacted First accessed variable
    when Starting Address = 0
    %SW138
    Modbus Discrete Input Offset
    FC02 Read Discrete Inputs %M[%SW138]
    %SW139
    Modbus Coil Offset
    FC01 Read Coils
    FC05 Write Single Coil
    FC15 Write Multiple Coils
    %M[%SW139]
    %SW140
    Modbus Input Register Offset
    FC04 Read Input Registers %MW[%SW140]
    %SW141
    Modbus Holding Register Offset
    FC03 Read Holding Registers
    FC06 Write Single Register
    FC16 Write Multiple Registers
    FC22 Mask Write Register
    FC23 Read/Write Multiple Registers
    %MW[%SW141]

    The base address is the number of %M or %MW which correspond to Modbus address 0000.
    Examples:
      • %SW140 contains 1000. When the Modbus server receives a Read Input Registers request (code 04) with starting address equal to 0000, it returns values from %MW1000.

        %SW138 contains 0001. When the Modbus server receives a Read Discrete Inputs request (code 02) with starting address equal to 0000, it returns values from %M1.

    Warning : it is mandatory to set the value of those 4 words with Initial value mechanism, rather than writing in an application section. Otherwise, upon PLC Init command or setting %S0 to 1 or downloading application, the 4 system words are set to 0000 and the behaviour of Modbus server is wrong.
    Causes and Fixes

    This information will be included in UnityPro V7.00 Help on line documentation

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