DF/1 in a World of Acronyms

We live in a world of acronyms. They’re all around us and that will never change.

My brother in-law that cardiologist has his own. NASA, an organization with an acronym for its name has as many of them as anybody. Policemen, firemen, priest and yes, even strippers use acronyms.

There’s a three letter one I want to focus on today. That one is DF/1. It’s interesting that when I pulled out my old DF/1 manual from sometime in the 80’s and there is no definition of DF/1. Every AB manual has a section called “Abbreviations”. The DF/1 manual abbreviation section doesn’t list DF/1. I can only presume it means “Data File” but it maybe it means “Darn Fun 1”. Possibly the engineers like it so much they presumed there would be a Darn Fun 2 or 3 or more.

DF/1 is really misunderstood. Most people think it is a higher level data protocol. That somehow it has some commands that move mysteriously move data in and out of a PLC. No, that’s not true. Not true at all.

DF/1 is simply a link layer protocol that moves a packet from point A to point B. One of the two points is an Allen-Bradley PLC.

In many ways, it is identical to the IP protocol on the internet. The IP protocol just moves packets, usually TCP packets, from a source to a destination. Nothing fancy, it’s a truck that moves things from one place to the other.

So, what does it move? What is this mysterious packet that DF/1 is moving in and out of PLCs?

Well that would be the PCCC messages. Ah, yes, another acronym. This one means Programmable Controller Command and Control messages. This is a message that indicates a read or a write of some File (remember old PLCs from AB use Files as their internal data structure) in the PLC. So, just like IP, DF/1 is moving packets that contain another protocol that actually contains actionable messages.

A couple of other things about this guy that you should know. Most of the ports on the PLC that are used for DF/1 communications are RS232 so you have to be pretty close (within 50 feet) to get this to work without problems. It was used a lot in the old days for sending new programs into the PLC.

And lastly, as with everything else there are two types of DF/1 communications. There’s Full Duplex which means that the two (and only two) devices are sending data at the same time. That’s of course more efficient than Half Duplex in which you have multiple devices on a RS485 link and one device talks while all the other listen.

Later this year we’ll have a suite of DF/1 product available. Until then you can keep checking the Gateway Product Catalog page for more information on all our products.