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