Wago Modbus - Port name + Address > 65535

  • Hello,

    I'm working with a Wago PFC-200 and have two modbus questions:

    1. The modbus registers (Kohler generator, decisionmaker 3500) start at 400000. This is above the 'allowed' input of max 65535.
      If i configure the .rio file directly, i'm not sure if the register value is truncated or not. I have not been able to make the modbus work for this generator but have had it work with another device.

    2. I found out that the PFC200 has serial 3 ports in de /dev folder.
      ttyO0, ttyO1, ttyO2. The serial port on the PLC (750-8203) is ttyO0.
      I have a 750-652 Serial (232/485) module that does not appear in the supported modules for Rexygen and i have installed it as the last module (before the end module) to be sure it would not interfere with the module order/index.

    How can i find out what the port name is for the new module ?


  • Hi Richard,
    numbering of Modbus registers can be confusing. There are four independent tables of data in each Modbus Slave device, see e.g. http://www.simplymodbus.ca/FAQ.htm#Command

    In REXYGEN, all four tables are accessed separately and are numbered from 0.

    Some vendors follow the simplified numbering model and use the offset as pointer to the corresponding table. In your case, you will be reading Holding register (offset 400000) number 0. If you have a link to Kohler Modbus register map, I can take a closer look.

    I'll create a separate topic for the other question.

    Best regards,

  • Thanks again.

    I suspected this but working with the 750-652 i could not make anything work.
    Using the built-in serial port it works fine. 400001 is indeed 1.

    The 400000* threw me off since some modbus RO registers for other devices start with a 4 but then look like 4001 or 41001.

  • Hi Richard,
    there are various "simplifications", using various offsets to distinguish between the four tables of data. For Holding registers, the most typical is offset of 40000, but as you already noticed, this is no standard.

    Best regards,

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