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.
The maximum allowed differential voltage is
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.
The DE-9 is the de facto industry standard connector for RS-422.
The industry standard pinout for RS-422 in a DE-9 connector is shown in the table below3.
|1||TXD-||Transmitted data -|
|2||TXD+||Transmitted data +|
|3||RTS-||Request to send -|
|4||RTS+||Request to send +|
|6||RXD-||Received data -|
|7||RXD+||Received data +|
|8||CTS-||Clear to send -|
|9||CTS+||Clear to send +|
The D-Subminiature DB-25 connector is another popular choice for carrying RS-422 signals.
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|
|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|
Many RS-422 transceivers also support RS-485.
|Manf. Part Num.||Manf.||Protocols||\(V_S\)||Data Rate||Package||USD, 100||Comments|
|Renesas||RS-422, RS-485||3-3.6V||40Mbps||DFN-10||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. ↩︎
ActiveXperts. RS422 Serial Port Connector Pin Layout. Retrieved 2022-02-06, from https://www.activexperts.com/serial-port-component/tutorials/pinout422/. ↩︎
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