DH+ and Remote IO

They’re old and gray. Hunched over. Slowly shuffling along. Nothing like the hot new guys that we’ve come to know over the last 10 years or more. But these are the guys we cut our teeth on. The guys we’ve argued with, cussed at and if not loved, at least have come to honor and respect.

Of course I’m talking about DH+ and RIO. The old stalwarts of Allen-Bradley industrial automation. Maybe they’re not as old as the ageless Modbus but they sure have the wrinkles and weather-beaten skin befitting their age.

For you young pups, I’ll stop now and go through a bit of history. Long ago, when PLCs were a new thing there were lots of discussions and theories about how to network sensors and actuators. Those were the days of proprietary systems. Customers put a line in and they bought all Allen-Bradley or all TI, or all Westinghouse or all Modicon (long before Groupe Schneider).

Everything used a proprietary and “secret” interface. Only close partners could be part of the system.

And it was great. Really great. FOR THE VENDORS THAT IS. Not so much for the customers. The customers just sucked it up, bent over and paid to play with the system they were given. Very little choice in those days. But, to be fair, there wasn’t near as much automation as there is today.

In those days, it was a lot of switches, some early Motor Controls, a few valves and some photoeyes. Not a lot to network together.

This is the environment where AB introduced DH+ and RIO and literally created industrial networking!

DH+ is a peer network with a floating master. The bus master floats between different nodes. When a node get mastership it can send messages.

The physical layer is proprietary with differential signaling. That means that bits are passed by the difference in potential between the two signal lines. You can do a network with a lot of nodes and up to 10,000 feet long. Each drop can be up to 100 feet so you can cover a huge area with it. At its heart, it uses PCCC, the Programmable Control language that all AB PLCs understand.

Remote IO is similar to DH+ but with a more defined purpose. RIO simply moves I/O points from remote racks to a central processor. DH+ is more of a generic interface than RIO.

So, why do we need to bother with these antiques? Can’t we just forget them and go on? No, the answer is an EMPHATIC NO.

There are hundreds and hundreds if not thousands of miles of DH+ and RIO networks around the US [yes, primarily the US since that is where AB is the strongest and where the longest history of networking devices is]. These networks aren’t going to be ripped out just because of the gray hair and pearl handled, coffee stained cane. People need to keep these things running.

Just yesterday I talked to a major manufacturer that wanted to know how they could keep support for their DH+ and RIO products well into the conceivable future. Why, because they have lots and lots of existing applications that they don’t want to rip out. And it’s that way all over the place.

So, if you’re a vendor of sensors and actuators it might be tempting to dismiss these old guys but don’t be so quick. There are sales out there. And since a lot of your competitors won’t support this stuff you can get higher than normal margins for it.

It’s a good place to be!

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