RS-422 Protocol

Caution

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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.

Speed	Max. Distance
10kbps	1200m2
10Mbps	10m

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.

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

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

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

Pin Num.	Name	Description	Pin Num.	Name	Description
1	EGND	Earth ground	14	TXD+	Transmitted data +
2	TXD-	Transmitted data -	15	CLK-	Transmit clock -
3	RXD-	Received data -	16	RXD+	Received data +
4	RTS-	Request to send -	17	CLK+	Receiver clock -
5	CTS-	Clear to send -	18	-	Unassigned
6	DSR	Data set ready	19	RTS+	Request to send +
7	GND	Logic ground	20	DTR-	Data terminal ready -
8	DCD-	Data carrier detect -	21		Signal quality detect
9		Reserved	22	-	Unassigned
10		Reserved	23	DTR+	Data terminal ready +
11	-	Unassigned	24	CLK+	Transmit clock +
12	DCD+	Data carrier detect	25	CLK+	Receiver clock +
13	CTS+	Clear to send

Parts

Many RS-422 transceivers also support RS-485.

Manf. Part Num.	Manf.	Protocols	(V_S)	Data Rate	Package	USD, 100	Comments
ISL3179EIRZ	Renesas	RS-422, RS-485	3-3.6V	40Mbps	DFN-10	$2.48	Also supports RS-485.

Footnotes

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. ↩ ↩²

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. ↩