DF1, Another Acronym

PETAI had a friend that once that worked at NASA for a short time. He struggled in that job. Not because of the technology but because of the acronyms. NASA speaks acronyms. He went to meetings where he understood almost nothing anyone was saying. They used sentences like “We have to get the AIC into the EAW by MSI or JKR will be in trouble.” It might have just as well been Chinese or Russian. It literally took him years to assimilate the NASA terminology.

We have some of that in industrial automation, of course. The Rockwell and Allen-Bradley terms are, I’m sure, difficult for newbies to assimilate. AB has product names like CompactLogix, MicroLogix, and PLC 540/E. There are products like RsNetworks and RsLogix. And then there are the technologies like EtherNet/IP, DeviceNet, Modbus, and OPC UA. I think we often fail at helping new people sort all this out. I admit, it was much easier for me when I started: there was a lot less technology then. It was a long time ago—my first company had a team of scribes to record production numbers on tablets (the stone kind).

One of the terms I know that confuses a lot of people is DF/1. Few people really understand it and the term PCCC (Programmable Controller Command and Control), which is closely associated with it. PCCC is the protocol mechanism for sending/receiving data. DF/1 is the link layer protocol that moves PCCC messages from one node to another. Here’s a quick guide to these two terms:

  • Legacy AB PLCs, AB network tools, and some vendor products, like RTA’s products that read and write AB PLC data table, build PCCC messages. These messages are highly specific to a particular programmable controller and its data table configuration. PCCC, unlike DF/1, is not a trivial protocol.
  • PCCC messages are used to request services from a protocol controller; usually reading or writing data table values.
  • DF/1 simply moves a packet of data from one place to another. DF/1 has no commands. It has no knowledge of the contents of its packets.
  • DF/1 is a serial communications protocol that operates at baud rates from 9600 baud to 56K baud. 19.2K is most typical.
  • DF/1 has a full duplex version operating over RS232 in which messages can move simultaneously in both directions between two nodes. You can think of this as high performance messaging.
  • DF/1 has a half-duplex version in which multiple nodes can exist on an RS485 network. In the half-duplex implementation, a master node can send and get messages from up to 32 other nodes.

DF1/ uses RS232 for full duplex communications and RS485 for half-duplex communications. In Full duplex communications, two PLCs can simultaneously send messages over the link in both directions. In half-duplex communications, a DF/1 Master node sends messages to a specific node that must respond to that message. The DF/1 message includes a node address that identifies the destination node.

DF/1 is very important in a lot of legacy applications. There are still millions of legacy programmable controllers that use DF/1 and PCCC communications. To keep these programmable controllers viable, RTA has added DF/1 Master communications to its line of gateways. You can now move data in and out of the data tables of the old PLCs, SLCs, Micrologix, and some of the Logix family of controllers using front port communications with the RTA gateways.

For example, if you have a meter that uses Modbus RTU serial communications, you can access that meter from a legacy PLC using a DF/1 to Modbus gateway. Instead of Modbus, maybe you have some ASCII data, or an Ethernet TCP display. You could move your ASCII data into a legacy PLC with the 460DFMA. You could write an Ethernet TCP display device from you old PLC5 from with the 460DFTCP.

There are many ways to use the DF/1 technology to maintain and extend the capabilities of your legacy Allen-Bradley programmable controllers. Contact our inside sales folks at 262-436-4999 to see how we can solve your application problem.

We’ll keep working on making your inaccessible data accessible.
John

P.S.
For those of you with the brain storage to waste DF1 stands for Allen-Bradley’s data-link layer protocol that combines features of subcategories D1 (data transparency) and F1 (two-way simultaneous transmission with embedded responses) of ANSI x3.28 specification. A questionable case for an acronym but just as confusing.

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