ITU X-Series Serial Interfaces » History » Version 4
laforge, 01/30/2023 06:32 PM
| 1 | 3 | laforge | {{>toc}} |
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| 3 | 1 | laforge | h1. ITU X-Series Serial Interfaces |
| 4 | |||
| 5 | h2. X.20 (Interface between DTE and DCE for start-stop transmission services on PDN |
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| 6 | |||
| 7 | X.20 defines an asynchronous serial interface consisting of a Transmit and Receive circuit (plus their associated ground/common return). |
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| 8 | 2 | laforge | |
| 9 | * Specification available from https://www.itu.int/rec/T-REC-X.20-198811-I/en |
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| 10 | * Originally specified in 1972, amended in 1976, 1980, 1984 and 1988. |
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| 11 | 1 | laforge | |
| 12 | 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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| 13 | |||
| 14 | * electrical characteristics can be X.26, X.27 or V.28 |
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| 15 | * connector: 15-pole ISO 4903 |
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| 16 | * characters for call control: 7-bit IA5 alphabet with even parity |
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| 18 | 3 | laforge | h3. Call control |
| 19 | 1 | laforge | |
| 20 | Call control is used to |
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| 21 | * request outbound calls (and provide status from the network) |
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| 22 | * indicate inbound calls (and accept or refuse them by the user) |
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| 23 | |||
| 24 | 3 | laforge | h4. Call control state machine |
| 25 | 1 | laforge | |
| 26 | !x20_figure_a1.png! |
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| 27 | |||
| 28 | 3 | laforge | h4. Call control time chart |
| 29 | 1 | laforge | |
| 30 | !x20_figureb1.png! |
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| 31 | 3 | laforge | |
| 32 | h2. X.20bis (Use of DTE designed for async duplex V-series modems on PDN) |
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| 33 | |||
| 34 | This interface has significantly more circuits than X.20: |
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| 35 | 1 | laforge | |
| 36 | 4 | laforge | |_.Number|_.Designation|_.Comment| |
| 37 | 3 | laforge | |102|Signal ground / common return| |
| 38 | |103|Transmitted data| |
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| 39 | |104|Received data| |
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| 40 | |106|Ready for sending| |
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| 41 | |107|Data set ready|ON: ready for data; OFF: DCE clear indication/confirmation| |
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| 42 | |108/1|Connect data set to line|ON: Call request for direct call/call accepted; OFF: DTE clear request/confirmation| |
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| 43 | |108/2|Data terminal ready|see above| |
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| 44 | |109|Data channel received line signal indicator| |
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| 45 | |125|Calling indicator|ON indicates _incoming call_| |
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| 46 | |141|Local loop back| |
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| 47 | |142|Test indicator| |
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| 48 | |||
| 49 | * electrical charactreristics acccording to V.28 |
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| 50 | * connector: 25-pole ISO 2110 |
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| 51 | |||
| 52 | h2. X.21 |
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| 53 | |||
| 54 | See [[X21]] |
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| 55 | |||
| 56 | |||
| 57 | h2. X.21bis (Use of DTE designed for sync duplex V-series modems on PDN) |
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| 58 | |||
| 59 | * electrical charactreristics acccording to (either) |
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| 60 | ** V.28 / 25-pole ISO 2110 |
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| 61 | ** X.26 / 37-pole ISO 4902 |
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| 62 | ** V.35 / 34-pin ISO 2593 (for 48 kbps) |
