Thursday, July 10, 2008

Quick Data Comms to AB PLC

One of my readers was asking for a quick way to talk via Ethernet to a Rockwell AB PLC.

You can actually talk to a ControlLogix by only understanding TWO (2) different packets, each with a response, so four packets I guess. The problem is this uses "UCMM" style communications, which the PLC has very limited resources for. Said another way, the CIP Connected Messaging or I/O production both include an inherent allocation of resources, while the UCMM is designed to be used ONLY to setup such pre-allocated resources.

So yes, you can use the information below to create a literal quick-n-dirty solution, and if you talk more than a few times a second you might start to interfer with other communications to the PLC (which is not a good thing!) However you could treat this as a proof of concept, and then work to do the communications more fully per the ODVA specs.

Here is the PDF of how to read/write to the PCCC Object in a Logix Processor here.

1 comment:

Lynn August Linse said...

Shane M had a question: Where are the actual integer file read response in the PDF.

The read request is in section 3.1, bytes 64 to 73, which is a PCCC or DF1-like command defined in the Ab DF1 manual.

The read response is in section 3.2, bytes 52 to 63, with the first integer in bytes 58 to 59.

But of course use common-sense, you'll need to confirm the response form appears 'as expected' or you'll just blindly assume garbage is valid.