ITU X-Series Serial Interfaces » History » Revision 3
Revision 2 (laforge, 01/30/2023 06:18 PM) → Revision 3/4 (laforge, 01/30/2023 06:31 PM)
{{>toc}} h1. ITU X-Series Serial Interfaces h2. X.20 (Interface between DTE and DCE for start-stop transmission services on PDN X.20 defines an asynchronous serial interface consisting of a Transmit and Receive circuit (plus their associated ground/common return). * Specification available from https://www.itu.int/rec/T-REC-X.20-198811-I/en * Originally specified in 1972, amended in 1976, 1980, 1984 and 1988. 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 h3. h2. 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) h4. h3. Call control state machine !x20_figure_a1.png! h4. h3. Call control time chart !x20_figureb1.png! h2. X.20bis (Use of DTE designed for async duplex V-series modems on PDN) This interface has significantly more circuits than X.20: |_.Number|_Designation| |102|Signal ground / common return| |103|Transmitted data| |104|Received data| |106|Ready for sending| |107|Data set ready|ON: ready for data; OFF: DCE clear indication/confirmation| |108/1|Connect data set to line|ON: Call request for direct call/call accepted; OFF: DTE clear request/confirmation| |108/2|Data terminal ready|see above| |109|Data channel received line signal indicator| |125|Calling indicator|ON indicates _incoming call_| |141|Local loop back| |142|Test indicator| * electrical charactreristics acccording to V.28 * connector: 25-pole ISO 2110 h2. X.21 See [[X21]] h2. X.21bis (Use of DTE designed for sync duplex V-series modems on PDN) * electrical charactreristics acccording to (either) ** V.28 / 25-pole ISO 2110 ** X.26 / 37-pole ISO 4902 ** V.35 / 34-pin ISO 2593 (for 48 kbps)
