I have extensively used RS232 transceivers in many projects and have fairly good understanding of UART communication using RS232 transceivers. MAX232 and MAX3232.

Now I have to build my understanding about RS422 and RS485 transceivers. So far I could not build crisp understand of the two as to when is one better than other. One thing that is clear to me is that RS422 use uni-directional transceivers for RX and TX differential lines while RS485 uses tri-state-able bi-directional transceivers.

For RS232 I know its always full-duplex but for RS422 and RS485 both of them are at some places explained a full-duplex while at other places as half-duplex buses.

What can we say with certainty about it for these busus?

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  • \$\begingroup\$ Is RS-422 ever half-duplex? I've never seen it as half-duplex. I thought it was just multi-drop or full-duplex. \$\endgroup\$ – DKNguyen Jul 18 '19 at 5:40

The RS422 and RS485 standards only specify the electrical requirements. Both can be full-duplex but isn't a requirement of either specification. They are also similar enough that you can interoperate them in some situations.

Now the big difference, which you already noted is that RS485 uses a tri-state system. This means you can have multiple transmitters sharing the single two wire bus, switching individual nodes as needed. In this configuration full-duplex probably isn't necessary or desirable.

In comparison RS422 can only have one driver per wire pair, but the specifications allows for 10 receivers.

If you're only using them for point-to-point communication, there really isn't much difference.

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  • \$\begingroup\$ Just like MAX3232 is a good transceiver chip for RS232, can you name ones for RS422 and RS485 transceivers? \$\endgroup\$ – alt-rose Jul 18 '19 at 6:55
  • \$\begingroup\$ @alt-rose Asking for recommendations of specific chips is off-topic for this site. Such answers do not have lasting value, because better chips come along all of the time. \$\endgroup\$ – Elliot Alderson Jul 18 '19 at 11:42
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    \$\begingroup\$ Depends on the application as well. A half-duplex RS422 transceiver that's great for multi-point topology probably isn't great if you wanted a full-duplex point-to-point solution. If you're happy with Maxim chips they have a whole range of them. (MAX485) \$\endgroup\$ – hekete Jul 18 '19 at 14:30

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