Do you remember the term data highway? That’s what they called the internet way back when. It’s also what Allen-Bradley decided to call one of its early proprietary networking protocols: Data Highway, shortened to DH. And believe me when I tell you, when it was introduced, it was stunning and revolutionary. Now, not so much. It’s kind of ho-hum, obsolete and nostalgic.
Data Highway was developed by Allen-Bradley in the glory days of programmable controllers (the 1970s) as a proprietary network connecting A-B PLCs, remote IO systems, PCs and HMIs. Prior to DH, every sensor and actuator required an I/O point on a PLC card and a minimum of two wires from the device to the I/O connection on the PLC input card. Data Highway was one of the first technologies to move field connections closer to the machine and out of the control cabinet.
While it wasn’t the first I/O bus in manufacturing, it was the first I/O bus supported by Allen-Bradley programmable controllers (PLC2s and PLC3s at the time). Data Highway reduced wiring expenses, simplified I/O architectures and made the term “blue hose” ubiquitous in manufacturing plants around the world.
The technology was protected under trade secret and patent protection until the late 1990s, an era that was anything but “open source.” In those early days, the business plan was to develop some proprietary technology, patent it, lock your customers into it and profit from it, competition free, for many years. Of all the DH technologies, DH+ was the one that was adopted most widely. (DH-485 achieved some success connecting connected Allen-Bradley’s SLC programmable controllers.)
DH+ is still used in many legacy manufacturing applications. Like much of the legacy technology in older manufacturing plants, there is no real incentive to replace it. Replacing controllers, wiring, I/O cards and the rest means downtime, expensive upgrades, integration costs, documentation and, often, retraining a maintenance staff. All that expense to make the same product means that technologies like DH+ will continue to exist until there is a more pressing reason to upgrade the entire manufacturing cell or manufacturing line.
Today, DH+ enabled PLCs and devices are highly coveted. Manufacturers using DH+ in their systems are unable to buy new controllers, devices and I/O from Allen-Bradley, so there’s a strong secondary market for those legacy devices.
If you’re looking to take a road trip down memory lane, there are few better avenues than the old Data Highway. It’s a great reminder of what used to be a revolutionary factory floor protocol. If you’re already using DH+, eventually you’ll have to make the hard and expensive decision to upgrade. But until then, you can scour the secondary market for what you need. And if you’re looking for more information about DH+, our technology page goes even deeper into this legacy protocol, comparing it to EtherNet/IP and featuring sections on Characteristics of the Physical Network Architecture, Link Layer Operation, Application Layer Messages.