A Made IT project

X.21

General

Voltages:	+/- 0.3Vdc
Speeds:	Max. 100Kbps (X.26)
Speeds:	Max. 10Mbps (X.27)

The X.21 interface was recommended by the CCITT in 1976. It is defined as a digital signalling interface between customers (DTE) equipment and carrier's equipment (DCE). And thus primarally used for telecom equipment.

All signals are balanced. Meaning there is always a pair (+/-) for each signal, like used in RS422. Electrally the X.21 signals are the same as RS422, so please refer to RS422 for the exact details.

Pinning according to ISO 4903


Sub-D15 Male	Sub-D15 Female

Pin	Signal	DTE	DCE
1	Shield	-	-
2	Transmit (A)	Out	In
3	Control (A)	Out	In
4	Receive (A)	In	Out
5	Indication (A)	In	Out
6	Signal Timing (A)	In	Out
7	Unassigned
8	Ground	-	-
9	Transmit (B)	Out	In
10	Control (B)	Out	In
11	Receive (B)	In	Out
12	Indication (B)	In	Out
13	Signal Timing (B)	In	Out
14	Unassigned
15	Unassigned

Functional Description
As can be seen from the pinning specifications, the Signal Element Timing (clock) is provided by the DCE. This means that your provider (local telco office) is responisble for the correct clocking and that X.21 is a synchronous interface. Hardware handshaking is done by the Control and Indication lines. The Control is used by the DTE and the Indication is the DCE one.

Cross-cable pinning

X.21 Cross Cable
X.21	X.21
1	1
2	4
3	5
4	2
5	3
6	7
7	6
8	8
9	11
10	12
11	9
12	10
13	14
14	13
15

Resources: