RS-422 – The Other Bus-meat

I encountered an RS-422 bus for the first time recently and it threw me through a bit of a loop! The device was a Modbus/RTU slave device and so I anticipated a RS-485 connection as per usual, but that was not the case. It’s the same cable, and the same essential signalling standard, but if you try to interface with RS-422 as if it’s RS-485 you’re going to have a bad time.

What’s RS-422?

It’s similar to RS-485, but it has some key differences:

  • Only one device gets to be the big talker, and everyone else has to listen. If you have multiple receivers, they can’t talk back. If you only have two devices, no problem. They can actually talk at the same time, which is faster than your usual half-duplex RS-485.
  • It can have one driver (talker) and 10 receivers, RS-485 can have 32 “unit loads” which can mean many more than 32 controllers (not many manufacturers would recommend it, though)
  • It’s a four-conductor bus. Just like RS-485 has a single balanced pair, RS-422 has two balanced pairs: one for transmission, and the other for reception.
  • You wire the driver (talker)’s transmit to the receive terminals of all of the slave devices

Some RS-485 adapters support RS-422, but don’t assume it.

Based on the standard, RS-485 can be “full-duplex” too. An extra pair of wires allows a bus to send and receive at the same time by using four wires, but it’s unnecessary and rare for RS-485.