Ladder Logic 309: Allen-Bradley Produced-Consumed Tags (Video)

Today’s post is a continuation on Jason Lafever’s questions on PLC to PLC communications. PLCs often have a need to send messages to each other, and all have some method of directly sending a message to read or write tags from processor to processor. The previous post on the MSG (Message) instruction from Allen Bradley is an example of this. Siemens also has a large selection of these types of instruction, generally called “Put” and “Get” or Send (Snd) and Receive (Rcv).

The Allen-Bradley RSLogix5000 platform has made this even easier by allowing processors to share tags. Part of the reason this works is that the Allen-Bradley ControlLogix platform works on a producer-consumer model; for instance, input cards “produce” information on a timed basis that the processor then “consumes”, and the CPU “produces” information similarly for the output cards. This has an interesting effect on the way their newer PLCs scan, as described in the Ladder Logic 105 article on Scanning.

In order to ensure proper timing of I/O and communications, an RPI (Requested Packet Interval) is defined in the hardware setup for each card. This time needs to be designated for the produced tags also, how often tags will be updated for consumption by other processors.

The following video shows how these tags are programmed using Allen-Bradley’s RSLogix Studio 5000 software:

I also have some exciting news about getting into a new training facility. I spent the last couple of weeks in Miami working on the CIP Project I described in my last post, and while I was gone my wife found an appropriate place for my new training facility. It will be quite different from the last one that was wiped out in the tornado, I will show some pictures in a later post.

Until then, I hope you enjoy this video and can get some helpful information from it!


Electrical Engineer and business owner from the Nashville, Tennessee area. I also play music, Chess and Go.