X.21 Pinout

X.21 Description

The X.21 interface [an ITU-T spec] is normally found on a 15-Pin D Sub connector.
The pinout for the 15-pin connector is shown below.
X.21 used a state-driven protocol running full duplex from 9600bps to 64 Kbps.
Although it might be found at speeds of up to 10 Mbit/s over a 1200m cable.
The X.21 interface is out-dated, first being introduced in the mid 1970's.
The X.21 interface may also be found on other connector sizes.

Basically X.21 uses TIA423 for asymmetric signaling [X.26], and TIA422 for symmetric signaling lines.
The X.21 interface is obsolete, and may also be referred to as just X21, although RS422 & RS423 are not.
X.21 is found on a 15-pin Dsub connector, see data link below.

DB15 pin locations and connector shape
15 Pin Dsub Connector

X.21 Signal Descriptions and Pin Out

However the physical pin out depends on the interface type: X.26 or X.27.

15-Pin Dsub X.21 Signals
Pin name Pin Description
1 Shield
2 Transmit A
3 Control A
4 Receive A
5 Indication A
6 Signal Timing A
7 Terminal Transmit Clock A
8 Signal Ground
9 Transmit B
10 Control B
11 Receive B
12 Indication B
13 Signal Timing B
14 Terminal Transmit Clock B
15 Not Used

Manufacturers of
D-Subminiature Connectors [D-sub Connectors].
15-pin D-Subminiature Mechanical Drawing, with pin locations.

Refer to the Buses icon below for descriptions of all other interface buses, including other cable buses.
Refer here for a listing OEM Manufacturers listed in alphabetic order.

As an engineer this cable interface pre-dates my experience, and have never had the opportunity to interface to it.
So unlike other interface standards listed on this site only the pin out table is provided
Along with the warning not to use this interface unless it's required by some other component.

PC motherboard

Distributor rolodex Electronic Components Electronic Equipment EDA CDROM Software Engineering Standards, BOB card Cabled Computer Bus Electronic Engineering Design Table Conversion DB9-to-DB25.
DistributorsComponents Equipment Software Standards Buses Design Reference

Modified: 3/04/12
Copyright © 1998 - 2016 All rights reserved Larry Davis