RS422 vs 485

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The_Fugitive
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Joined: Sun Sep 23, 2018 5:54 pm

RS422 vs 485

Post by The_Fugitive » Sun Sep 23, 2018 5:57 pm

Hi all,

In communicating between 1 transmitter and 1 receiver, is there any advantage to be gained by using RS-485 rather than RS-422? From what I understand, the only advantage 485 has over 422 is a greater number of nodes?

Any help would be much appreciated.

Cheers

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phalanx
Non-SFE Guru
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Joined: Sun Nov 30, 2003 8:57 am
Location: Candia, NH

Re: RS422 vs 485

Post by phalanx » Mon Sep 24, 2018 8:13 am

RS422 is spec'd to have a single always on transmitter driving up to 10 receivers on a single differential pair of wires where RS485 can have up to 32 bidirectional devices on a single differential pair of wires that are high impedance when not active on the bus. You can also operate both configurations in a full duplex mode by adding a second differential pair so long as your transceivers support it.

Due to the increased load potential of RS485 and the bidirectional nature of their communications, their transceivers tend to be more robust in handling short circuits and other common issues that pop up in these types of systems. That said, most of the modern transceivers you will find in the market are built to the RS485 standards (or better) and are compatible with RS422 as well. There are many different variants out there for every possible combination you could think of. High bandwidth, low bandwidth, full duplex, half duplex, driver enable pins, slew rate control, fraction of unit load, etc., etc.

Keep in mind that RS422 and RS485 are electrical specifications and do not define a protocol for your data being transmitted. Typically asynchronous serial data is sent over them (UART) but they are not limited to that.

-Bill

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