X.21
The X.21 is an interface standardized by the ITU-T for data transmission in public dial-up data networks (e.g. Datex-L ). Due to its simple structure, it is often used for direct connections (point-to-point).
Recommendation X.21 includes connection control and diagnostic options via test loops. It defines the format of the dialing characters, time sequences and other procedures in the form of status diagrams . In the data transfer phase, the connected line is transparent for all synchronous data formats . Clock lines ( supplied by the network termination or generated by the end device itself) enable the end device at the interface to synchronize with the transmission network.
In terms of the interface lines, the X.21 is simpler than the V.24 or EIA-232 / RS232 interface: it typically has five functions (control / message line, send / receive data, clock) and comes with a 15 pin connector according to ISO 4903.
The function of these lines is defined by ITU-T Recommendation X.24 .
Possible electrical properties are described in ITU-T X.26 and X.27 . They are defined in such a way that a higher data transmission speed and a greater range are possible than with an interface according to V.24.