X.21 is an ITU-T serial interface for digital WAN connections using a DB15 connector. Used in European digital leased line connections and some ISDN interfaces as an alternative to V.35.
| Pin | Name | Function | Type |
|---|---|---|---|
| 1 | Shield | Cable shield | GND |
| 2 | T_A | Transmit data A | BUS |
| 3 | C_A | Control A | I/O |
| 4 | R_A | Receive data A | BUS |
| 5 | I_A | Indication A | I/O |
| 6 | S_A | Signal element timing A | BUS |
| 7 | B_A | Byte timing A | BUS |
| 8 | GND | Signal ground | GND |
| 9 | T_B | Transmit data B | BUS |
| 10 | C_B | Control B | I/O |
| 11 | R_B | Receive data B | BUS |
| 12 | I_B | Indication B | I/O |
| 13 | S_B | Signal element timing B | BUS |
| 14 | B_B | Byte timing B | BUS |
| 15 | GND | Signal ground | GND |
X.21 is an ITU-T serial interface for digital WAN connections using a DB15 connector. Used in European digital leased line connections and some ISDN interfaces as an alternative to V.35.