The X.21 interface is normally found on a 15-Pin D Sub connector. The pinout for the 15-pin connector is shown below. X.21 used a state-driven protocol running full duplex from 9600bps to 64 Kbps. The X.21 interface is out-dated, first being introduced in the mid 1970's. The X.21 interface may also be found on other connector sizes. Also refer to the RS422 bus page which uses the same signal. The X.21 interface is obsolete, may also be found as just X21.
| Pin Name | Pin Description |
|---|---|
| 1 | Shield |
| 2 | Transmit A |
| 3 | Control A |
| 4 | Receive A |
| 5 | Indication A |
| 6 | Signal Timing A |
| 7 | Terminal Transmit Clock A |
| 8 | Signal Ground |
| 9 | Transmit B |
| 10 | Control B |
| 11 | Receive B |
| 12 | Indication B |
| 13 | Signal Timing B |
| 14 | Terminal Transmit Clock B |
| 15 | Not Used |
Refer to the Buses icon below for descriptions of all other interface buses, including other cable buses.
Engineering Design Key words: X.21, ISO,CCITT, Balanced differential Cable Bus, Electrical Standard, Interface Standard, Specification, Electrical Interface, Semiconductor IC manufacturers, Physical Interface, Engineering Description, Characteristics, Device, X.21 pinout, pin out
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