IO Scanning - Modbus Function Code-23 Exception Error

[jouharn]

I am getting an error message in Wireshark when i was trouble shooting a communication issue. It is noticed that PLC is sending function code-23 data request and not accepted by Slave device. 

Function Code-23 request is not sending frequently. it is occurring very rarely.  is there a way to control the function codes in IO Scanning or to avoid this exception messages?

[MatthewM · Accepted solution]

When you say it is rarely using FC 23, is it mainly using FC 03 and FC 16, and then occasionally probing with FC 23 to see if it works?

Some devices (M221 for example), you only need one line, and it tests FC 23 first, and if rejected, it breaks it up into a FC 03 and FC 16 automatically.  Perhaps the M580 is doing the same too.

If you want to make it robust and not have FC 23, make one I/O scanning line for the read and one for the write (one 0 write size and the other 0 read size).  This will make it use FC03 and FC16 and you shouldn't see any FC 23.

[SEStig · Accepted solution]

This is the way.....

The M580 does use FC23 in preference to a separate read and write if it can in one line. Splitting into two lines stops this but consumes more resources. If the FC23 is rejected it will then revert to 3 and 16.

[jouharn · Accepted solution]

Thanks for your reply 

[jouharn]

Thank you 😊