ITU X-Series Serial Interfaces » History » Version 2
laforge, 01/30/2023 06:18 PM
| 1 | 1 | laforge | h1. ITU X-Series Serial Interfaces |
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| 3 | h2. X.20 (Interface between DTE and DCE for start-stop transmission services on PDN |
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| 5 | X.20 defines an asynchronous serial interface consisting of a Transmit and Receive circuit (plus their associated ground/common return). |
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| 6 | 2 | laforge | |
| 7 | * Specification available from https://www.itu.int/rec/T-REC-X.20-198811-I/en |
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| 8 | * Originally specified in 1972, amended in 1976, 1980, 1984 and 1988. |
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| 9 | 1 | laforge | |
| 10 | 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. |
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| 12 | * electrical characteristics can be X.26, X.27 or V.28 |
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| 13 | * connector: 15-pole ISO 4903 |
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| 14 | * characters for call control: 7-bit IA5 alphabet with even parity |
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| 17 | h2. Call control |
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| 19 | Call control is used to |
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| 20 | * request outbound calls (and provide status from the network) |
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| 21 | * indicate inbound calls (and accept or refuse them by the user) |
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| 23 | h3. Call control state machine |
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| 25 | !x20_figure_a1.png! |
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| 27 | h3. Call control time chart |
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| 29 | !x20_figureb1.png! |
