# ITU-TSS X.21

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

(at the DTE)
(at the DCE)

CORRECT?

15 PIN D-SUB MALE at the DTE (Computer).
15 PIN D-SUB FEMALE at the DCE (Modem).

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 |

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

Contributor: Joakim Ögren

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

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May be copied and redistributed, partially or in whole, as appropriate.
Document last modified: 2001-06-07