RS-422 Protocol

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Published On:

Feb 5, 2022

Last Updated:

Jun 16, 2026

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 multidrop (unlike RS-485, which is differential and allows for multiple drivers).¹ It is closely related to the less popular RS-423 protocol. It is also related to the popular RS-232 (single-ended) and RS-485 (differential) communication protocols.

Speed and Cable Length

Data rates up to 10 Mbit/s can be achieved with cables up to 12 meters in length. Longer cables can be used at the expense of slower transmission speeds. No maximum cable length is specified in the standard, but the annex. provides a chart which shows lengths up to 1200 m (1.2 km) with a speed of 90 kbit/s.

The relationship between the maximum data rate and cable length is shown in the graph below. The data from this graph is taken from RS-422 Annex A.1.²

The maximum RS-422 cable length as a function of data rate. The length is flat at around 1200 m for low data rates up to a knee at roughly 90 kbit/s, then falls off as the data rate increases, reaching 12 m at 10 Mbit/s.

The maximum allowed differential voltage is $\pm 6.0\unit{V}$ .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 below.³

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.

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. ↩
Wikipedia (2022, Jan 25). RS-422. Retrieved 2022-02-05, from https://en.wikipedia.org/wiki/RS-422. ↩ ↩²
ActiveXperts. RS422 Serial Port Connector Pin Layout. Retrieved 2022-02-06, from https://www.activexperts.com/serial-port-component/tutorials/pinout422/. ↩
Michael R. Sweet (1999). Serial Programming Guide for POSIX Operating Systems. Retrieved 2022-02-12, from https://www.cmrr.umn.edu/~strupp/serial.html. ↩