ITU X-Series Serial Interfaces¶

X.20 (Interface between DTE and DCE for start-stop transmission services on PDN¶

Originally specified in 1972, amended in 1976, 1980, 1984 and 1988.

X.20 defines an asynchronous serial interface consisting of a Transmit and Receive circuit (plus their associated ground/common return).

The real content of X.20 is how call control is signalled over the serial interface. This is achieved without additional signals by "playing" with continuous-zero and continuous-1 signals on the T and R cirrcuits.

• electrical characteristics can be X.26, X.27 or V.28
• connector: 15-pole ISO 4903
• characters for call control: 7-bit IA5 alphabet with even parity

Call control¶

Call control is used to

• request outbound calls (and provide status from the network)
• indicate inbound calls (and accept or refuse them by the user)

Call control state machine¶

Call control time chart¶