ControlLogix - Soft PLC Message Control

Dear All,


I would liket to ask for your help.
I have a L73S PLC which communicates with a Soft Siemens PLC by MSG. (I already got the system that construction) I use AOI for that, and it looks working well.
BUT, sometimes I notice an anomaly in the communication.
For example: Soft PLC send a constant vaule (30) to the L73S, the L73S interprets the sent data as 30, but sometime it changes to 0, then to 256, and then again to 30. That anomaly width (30-0-256-30) is maximum 150ms.
I made a trend in the soft PLC, but when the anomaly occured the trend show 30 continuously in the soft PLC. In the L73S the trend show the value changing at the same time.

The block does not indicate an error in the communication.



Do you have any idea what could be causing this error?

Welcome to the PLCTalk forum community !

Where did you get the AOI that you are using to communicate between the controllers ?

What you are describing sounds like an issue related to data parsing logic. I can't say what it might be without more details.

This sounds like a great opportunity to use Wireshark to capture that data on the wire and make sure that each message contains the same payload.

A thing that just occurred to me: in general, Siemens and Rockwell use a different byte order. At some point that logic in the ControlLogix will have to swap the bytes of the data.

It is a common mistake to make the source and destination of a SWPB command the same tag.

It works fine for the actual logic; the value is correct at the end. But if an HMI or other program element reads the tag value from memory in the middle of the data handling program, it can get the pre-byte-swap bytes and interpret it as the wrong number.