DH-485

Data Highway 485, multi-drop network for Allen-Bradley SLC 500 and MicroLogix PLCs. This term is essential for understanding communication protocols in industrial automation and PLC programming.

DH-485 is one of the protocol or networking concepts an automation engineer encounters as soon as a plant has more than one controller talking to a shared device. Data Highway 485, multi-drop network for Allen-Bradley SLC 500 and MicroLogix PLCs.

Under the hood, DH-485 is implemented at a specific position in the OSI model — typically physical, data-link, or application layer — and its behaviour determines whether downstream PLC scan cycles can stay deterministic. Engineers have to consider cycle-time impact, jitter, security, and brand support when comparing it against alternatives.

On the bench or in the field, DH-485 typically reveals itself through specific symptoms — timeouts, packet loss, addressing conflicts, or framing errors — that point to the specific layer at fault. Mature engineers learn the symptom-to-cause mapping for each protocol they touch.