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Serial Pinout Diagrams
 
X.21 Pinouts

General
 

 

Voltages:

+/- 0.3Vdc

Speeds:

Max. 100Kbps (X.26)

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

Sub-D15 Male

Sub-D15 Female

 

Pin

Signal

abbr.

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