Help on PLC communications


Thread Starter

marie o brien

On the advice recently regarding tightening up modbus addresses to get communications to work, I changed my modbus addressing for one of my PLC's. The system is now showing communications( as in when i turn my PLC to halt it shows fail on the DCS side and when its running it shows running on the DCS side) , so there is communications there. However, its not writing the values across for the temperatures i'm logging. I'm new to this and i havent any idea why it wont communicate the values . I definitely have all the parameters set up right (the baud rate etc) , and all the modbus addressess are allocated correctly , but the values just arent coming through. I'm reading and sending them as the right type as well (unsigned integer)
Please help.


Robert Dusza


Going along with the information from Frank, you can look at the responses using software like BreakOut-II ( This can be done with one or two serial ports on a laptop or PC. The info will be in Hex, but you convert the info to Decimal and read the error codes. I have done this a few years ago. It is a pain but can yield valuable info.

Let me know directly if you need more info.


Robert J. Dusza, Jr.
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You can check this communication by the debug dialog in PL7 software for PLC, this dialoge allow you to send the command by hexdecimal and when, received the return value by hexdecimal, after that there are 2 bytes to check the error you can use the help for PL7.

I hope you will find the solution.