THVD1424: What's the difference between RS-485 and RS-422?

Hello Peter,

You are correct that RS-485 is half duplex, and RS-422 is full duplex, but another significant difference is that RS-422 can only have one driver on each bus. While RS-485 can have multiple drivers, only one driver can be driving at a time. So, while you can use RS-485 transceivers for RS-422 interfacing, you have to be careful about having only one of the transceivers driving per bus. Generally, you cannot use RS-422 transceivers for Rs-485 interfacing simply because they aren't built for it, and there could be consequences in shorting the data busses together and dealing with the driver logic.

For the THVD1424, there should be no problem in using it as a RS-422 transceiver, as long as it is set up to follow the rules for RS-422 interfacing. If you have any other questions, please reach out!

Zach

An RS-422 bus cannot have more than one driver. It can have more than one receiver, but that is usually not useful; in practice, RS-422 almost always is point-to-point.

An RS-485 bus can have multiple drivers (but only one can send at the same time), and usually has multiple receivers.

An RS-485 bus needs termination resistors at both ends, so RS-485 drivers need more drive strength than RS-422 drivers. This is why you can use RS-485 drivers for RS-422, but RS-422 drivers do not work with RS-485. (The THVD1424 can be used for both RS-485 and RS-422.)

An RS-485 bus uses one connection (two lines) for many devices. With RS-422, one connection goes only in one direction, so if you want to communicate in both directions, you need full-duplex transceivers with separate drivers and receivers, so that you have two connections (four lines). (This is not the only way to use RS-422. For a unidirectional connection, you can use a single driver and a single receiver, i.e., you could use parts of half-duplex transceivers. It is also possible to use more channels, for example, the AM26C31 has four drivers, and the AM26C32 has four receivers.)

Hi Peter,

Basically, what Clemens said. RS-422 is almost always point-to-point, and while a bus can have multiple receivers, a bus can only have one driver. On the other hand, RS-485 can have each node alternate between driving and receiving, but only one node can be driving at a time.

If you have a system where you want multiple drivers, then RS-485 would be the better option. While there are full-duplex RS-485 options, most RS-485 transceivers can still only transmit or receive at any given point since only one node should be transmitting at a time. Choosing a full-duplex RS-485 option generally isn't the most helpful, which is why many stick with half-duplex with RS-485. Feel free to reach out with any other questions!

Zach

Hi Zach, 

Thanks for clarification.

Peter