Data Highway (DH+) Revisited

It’s been twelve years since I did an article on DH+. That’s a long time. And guess what, DH+ is still here. People are still using old SLC and PLC5 controllers and running that “blue hose” between those controllers.

So, for the majority of people in the industry that haven’t been around nearly as long as I have, this article will introduce you to this archaic old stalwart that you can probably find buried in your plant.

Prior to Data Highway, programmable controllers were standalone. They had inputs and outputs wired to them (wireless was just an idea). There was no controller-to-controller networking so PLCs could work together. You would just tie some I/O lines between the controllers, and they could talk to each other using discrete signals. Yes, Fred Flintstone was the control engineer in those days.

Modbus became the first real industrial I/O network. It was pretty revolutionary for its time. It used new-fangled RS-485 communications technology at speeds all the way up to 9600 baud.

Data Highway was introduced after Modbus and it was pretty impressive. It operated at a much higher baud rate, and for the first time, controllers could move a lot of data around the network. It was a huge seller. Hundreds of thousands of controllers were sold with DH+ networking technology, and a good bit of it is still around today.

Here’s a quick summary of DH+ basics:

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. The 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)

MEDIA – DH+ is implemented using baseband shielded twin axial cable. Baseband in this context is defined as all devices on the medium using a single frequency.

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 ACCESS – DH+ was one of the first industrial networks to use a token-passing protocol. Every device in the network knows its successor node, its successor’s successor node, and its predecessor. Messages are exchanged by moving the token from one node to its successor. When a node grabs the token, it is then able to transmit messages to other devices on the network. It keeps the token until it expires or passes it along when it’s done sending.

If a controller drops offline and disappears from the ring, the token is passed 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 links. New nodes join the network when current nodes find them during periodic searches for additional successors.

MESSAGE PACKET – DH+ uses the same protocol packet as DF/1, the PCCC (Programmable Controller Control and Command Language) protocol. PCCC messages are customized for each PLC model and used to access the data tables of other PLCs.

We’ll have more to say about DH+ in the coming days. Even today when we are all using EtherNet/IP and PROFINET IO, DH+ isn’t dead yet!