Hello,

I'm a newbie to modbus communication. I have a Festo PLC and have configured the Modbus TCP server and can connect to it using a freeware Modbus TCP client

Now I want to test the addressing. Apparently the addressing insde the Festo PLC is open and the user is free to choose his own addressing. I'm not sure if I have to write code in Codesys to map the IO and then address these addresses from the client? Or does Codesys provide a way to define this addressing?? Please advice or provide example if available.

Thanks,

-Dude

As a follow-up to my own post, I was able to map the Outputs on my Festo PLC to the Mobbus TCP address space. This is the format I used:

Output0 := PLC_PRG.awModbusData[0].0;

Output1 := PLC_PRG.awModbusData[0].1;

I can successfully switch on these outputs from my ModBus Client. However the same mapping does not work if I want to read in Inputs. Codesys gives the following error:

Error 4020: Operands of 'ST', 'STN', 'S', 'R' must be variables with write access or direct address.

I tried using direct addressing and symbolic names but does not work. Any ideas??

Thanks,

-Dude

Sounds like you did the following:

Input0 := PLC_PRG.awModbusData[1].0;

Input1 := PLC_PRG.awModbusData[1].1;

This would cause the error you described change it to :

PLC_PRG.awModbusData[1].0 := Input0;

PLC_PRG.awModbusData[1].1 := Input1;

Or you don't have awModbusData[1] configured as output instead as input this will not give it write access as well. Sounds like you might have one or both variables configured as inputs (readonly).

The CoDeSys error will give you a line number in your code. Try to copy the code to make it easier to help.

Hello,

Actually I had the inputs mapped like:

PLC_PRG.awModbusData[1].0 := Input0;

PLC_PRG.awModbusData[1].1 := Input1;

and it still did not work. Then somebody told me that "awModbusData" should be defined as a global variable. I did that and it worked like a charm. I would have never figured it out since the outputs mapped and worked correctly without declaring "awModbusData" as a global variable. Maybe you have a better idea.

Anyways, my problem is solved for now. Thanks for your effort.

Thanks,

-Dude