Will DF1 Live Forever? (Revisited)

DF1 is no longer entering middle age, it’s creeping up on retirement age. Anyone who remembers when it first came out would have to be well into retirement age themselves. People of that era had a sweet pension deal at Allen-Bradley when it was still owned by the Bradley family. Now, A-B is part of Rockwell Automation and pensions are almost unheard of.

But let’s stop and catch up on a few things before moving on.

DF1 is one of those terms that a lot of people use but few really know. It’s used to talk about the protocol for moving data in and out of Allen-Bradley PLCs, some drives and a few other products. But really, it’s just a link layer protocol. That means that its job is to move a message from point A to point B. Nothing more. Nothing less. It has absolutely nothing to do with the contents of the message. Just move it from here to there.

Where most people go wrong is to assume that DF1 also includes the messaging that allows a user to write a file (N7:200 or the like) and read a file. That messaging is not DF1, but rather something called PCCC, which has a number of meanings. The meaning for PCCC that seems most appropriate is Programmable Controller Command and Control language.

PCCC is the protocol that allows for reading and writing all the different kinds of file structures that one finds in the various types of Allen-Bradley PLCs. PCCC is encapsulated in a DF1 link layer message. So, the two of them work together. A user application builds a PCCC message and then hands it off to the DF1 layer that moves it from here, wherever “here” is, into the PLC.

Even today, more than eight years since this blog was originally posted, PCCC and DF1 are still very much around and very much integral to the communications of Allen-Bradley PLCs. DF1 and PCCC are as important to the operation of Rockwell Automation’s PLCs as EtherNet/IP, DeviceNet or anything else. The question comes down to: why?

It’s reliable. The code is done. It’s already there and tested. There’s an infrastructure built around it that is comfortable and soothing. Like those old slippers sitting in the closet, it just feels good. This feeling of comfort came up not too long ago with a customer with a serious issue. Not only does this customer have an old system that uses DF1, but they also modified how it works to make it more comfortable for everyone using it. Talk about creating a disaster!

They now need to take that old, modified DF1 code that is generated by this old legacy system they don’t want to touch and convert the messages it sends out (the modified DF1) into messages that a MicroLogix can understand (standard DF1, or to be more precise, PCCC).

To be fair, it wasn’t a hard project. It just required a day or so at the customer’s offices to create a viable gateway for them. But moving forward, solutions will become more and more difficult to find as this technology sunsets, especially now that A-B no longer makes PLCs with serial ports or supports DF1. At RTA, we do a lot of these “get me out of this jam” kinds of projects.

So, to answer the original question: will DF1 live forever? Probably not, but that’s said only because nothing lives forever. Father Time is undefeated. However, there are more than 20 million Allen-Bradley legacy PLCs in the world today. It might take another century before they’re all obsoleted, trashed and abandoned. So, DF1 (and PCCC) is going to be with us for a long, long time. And that’s okay, that old pair of slippers is still comfortable…until it’s not.

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