Common names: ITU-TSS (CCITT) X.21, ISO 4903
15 pin D-SUB FEMALE connector at the DTE (Computer)
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)