RS-422 PROTOCOL

RS-422 Protocol

Article by:
Date Published:
Last Modified:
Warning This page is in notes format, and may not be of the same quality as other pages on this site.

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)

dsub db 9 female male photo pin numbers
Figure 1. 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.

Table 1. Industry standard pinout for RS-422 on a DE-9 connector[2].
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.

dsub db 25 pin numbers
Figure 2. Pin numbers for the male DB-25 connector, looking at the "front" of the connector which mates with the female part.
Table 2. Industry standard pinout for RS-422 on a DB-25 connector[4].
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

ISL3179EIRZ

Renesas

RS-422, RS-485

3-3.6V

40Mbps

DFN-10

$2.48

Also supports RS-485.

References


Authors

Geoffrey Hunter

Dude making stuff.

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License .

Related Content:

Tags

comments powered by Disqus