RS-422 PROTOCOL
RS-422 Protocol
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1. 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 multidrop[3] (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 |
1200m[1] |
10Mbps |
10m |
The maximum allowed differential voltage is \(\pm 6.0V\)[1].
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.
2. Connectors
The DE-9 is the de facto industry standard connector for RS-422.
2.1. DE-9 (DB-9)
The industry standard pinout for RS-422 in a DE-9 connector is shown in the table below.
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 + |
2.2. DB-25
The D-Subminiature DB-25 connector is another popular choice for carrying RS-422 signals.
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 |
3. Parts
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. |
References
-
[1] 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/.
-
[3] 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.
-
[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

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