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pinout (распиновка)

ITU-TSS X.21 connector

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Common names: ITU-TSS (CCITT) X.21, ISO 4903

15 pin D-SUB FEMALE connector  at the DTE (Computer)
Pin Name Dir Description Type
1 n/a --- Shield Ground
2 T (A) --> Transmit (A) Data
3 C (A) --> Control (A) Control
4 R (A) <-- Receive (A) Data
5 I (A) <-- Indication (A) Control
6 S (A) <-- Signal Timing (A) Timing
7 B (A) <-- Byte Timing (A) Timing
8 G --- Ground Ground
9 T (B) --> Transmit (B) Data
10 C (B) --> Control (B) Control
11 R (B) <-- Receive (B) Data
12 I (B) <-- Indication (B) Control
13 S (B) <-- Signal Timing (B) Timing
14 B (B) <-- Byte Timing (B) Timing
15 unused
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.
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.

15 pin D-SUB FEMALE connector  at the DCE (Modem)

Note: Direction is DTE (Computer) relative DCE (Modem)

Источник: X.21 Page at Connectivity Knowledge Platform (Made IT)