Data Highway plus (DH+)

Data-Highway-Plus-(DH+)
Data highway is pretty ancient technology. It was designed and built by Allen-Bradley in the late 70s. I think it was the first network ever used by an Allen-Bradley Programmable Controller and maybe any Programmable Controller. There was a lot of this stuff deployed in the 80s and 90s. Hundreds of thousands of nodes. Big numbers.

 

It’s still around in lots of factories around the world. If you do any kind of integration it’s likely that you will need to either get into a DH+ network or replace parts of one. So, let’s dive into what exactly DH+ is.

 

ELECTRICAL INTERFACE – DH+ uses Transformer Coupled Differential Signals meaning that stations do not have to be at the same ground potential (That’s a really good thing). Two wires are used to carry data and that data is represented by voltage differences between the two wires; known as differential signaling. Because the data is encoded differentially, noise that is common to both wires is ignored. Signal Level for these differential signals is typically 8-12 volts peak to peak. The Baud Rate is 57.6 kilobaud with Half Duplex transmission (when one node transmits, all other nodes go into receive mode)

 

TOPOLOGY – DH+ uses trunk lines with drops. That means that there is a main cable and at points along this trunk there is a small drop line that attaches a device to the DH+ network.

 

MEDIA – DH+ used Baseband Shielded Twin Axle Cable. In this case baseband simply means that all devices on the media are using a single frequency.

 

MEDIA ACCESS – DH+ uses a proprietary token passing protocol. In this protocol, every device knows its successor, its successor’s successor and its predecessor. It receives a message that indicates that it has the token. When it has the token it can send one or more messages to other devices on the network. After sending the message it passes the token to its successor. If the successor disappears it passes the token to its successor’s successor. Failing that it has to find another successor node. This protocol is referred to as a link layer protocol as it manages and maintains the network link. New nodes join the network when current nodes find them during periodic searches for additional successors.

 

PACKET – What’s in the DH+ packet. That’s another old protocol, PCCC (Programmable Controller Control and Command Language). Those are also proprietary messages that PLCs use to access the data tables of other PLCs.

 

We have plans to create a DH+ Device Converter. If you’d like to influence how the thing works, hit the contact button from any one of our product pages and let me know what you think.

John