RS-422 Protocol

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Overview

RS-422 (a.k.a. TIA/EIA-422) is a serial, differential communication protocol used for transmitting data. RS-422 uses 2 wires for transmitting and 2 wires for receiving for full-duplex communication. 1 driver and up to 10 receivers can be connected per twisted pair, which is called simplex multidrop1 (unlike RS-485, which is differential and allows for multiple drivers). It is closely related to the less popular RS-423 protocol. It is also related to the RS-232 (single-ended) and RS-485 (differential) communication protocols.

SpeedMax. Distance
10kbps1200m2
10Mbps10m

The maximum allowed differential voltage is \(\pm 6.0V\)2.

A mark (binary 1) is transmitted with a negative differential voltage, a space (binary 0) is transmitted with a positive differential voltage. This is the same terminology and “inverse” polarity as RS-232 and RS-485.

Connectors

The DE-9 is the de facto industry standard connector for RS-422.

DE-9 (DB-9)

Pin numbering for the D-sub DE-9 (a.k.a. DB-9) female and male connectors.

Pin numbering for the D-sub DE-9 (a.k.a. DB-9) female and male connectors.

The industry standard pinout for RS-422 in a DE-9 connector is shown in the table below3.

Pin Num.NameDescription
1TXD-Transmitted data -
2TXD+Transmitted data +
3RTS-Request to send -
4RTS+Request to send +
5GNDGround
6RXD-Received data -
7RXD+Received data +
8CTS-Clear to send -
9CTS+Clear to send +

DB-25

The D-Subminiature DB-25 connector is another popular choice for carrying RS-422 signals.

Pin numbers for the male DB-25 connector, looking at the "front" of the connector which mates with the female part.

Pin numbers for the male DB-25 connector, looking at the “front” of the connector which mates with the female part.

Industry standard pinout for RS-422 on a DB-25 connector4:

Pin Num.NameDescriptionPin Num.NameDescription
1EGNDEarth ground14TXD+Transmitted data +
2TXD-Transmitted data -15CLK-Transmit clock -
3RXD-Received data -16RXD+Received data +
4RTS-Request to send -17CLK+Receiver clock -
5CTS-Clear to send -18-Unassigned
6DSRData set ready19RTS+Request to send +
7GNDLogic ground20DTR-Data terminal ready -
8DCD-Data carrier detect -21Signal quality detect
9Reserved22-Unassigned
10Reserved23DTR+Data terminal ready +
11-Unassigned24CLK+Transmit clock +
12DCD+Data carrier detect25CLK+Receiver clock +
13CTS+Clear to send

Parts

Many RS-422 transceivers also support RS-485.

Manf. Part Num.Manf.Protocols\(V_S\)Data RatePackageUSD, 100Comments

ISL3179EIRZ

RenesasRS-422, RS-4853-3.6V40MbpsDFN-10

$2.48

Also supports RS-485.

References


  1. Manny Soltero, Jing Zhang, Chris Cockril (2002, Jun). SLLA070D: RS-422 and RS-485 Standards Overview and System Configurations (Application Report). Retrieved 2022-02-06, from https://www.ti.com/lit/an/slla070d/slla070d.pdf↩︎

  2. Wikipedia (2022, Jan 25). RS-422. Retrieved 2022-02-05, from https://en.wikipedia.org/wiki/RS-422↩︎ ↩︎

  3. ActiveXperts. RS422 Serial Port Connector Pin Layout. Retrieved 2022-02-06, from https://www.activexperts.com/serial-port-component/tutorials/pinout422/↩︎

  4. Michael R. Sweet (1999). Serial Programming Guide for POSIX Operating Systems. Retrieved 2022-02-12, from https://www.cmrr.umn.edu/~strupp/serial.html↩︎


Authors

Geoffrey Hunter

Dude making stuff.

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