X.21bis is a physical-layer synchronous serial protocol which X.25 uses for communication with analog terminals. The physical X.25 connector is 15 pins with some pins used as NULL (unconnected). X.21bis uses only four wires for actual communication (one pair for transmit, one pair for receive) but uses several more wires for signalling and synchronization. X.21bis is therefore capable of full-duplex synchronous operation at speeds up to 19,200 kbps between two devices.
.21-bis Pinout
| Pin | Function | DTE/DCE |
| 1 | Protective Ground | |
| 2 | Transmit Data | |
| 3 | Control | |
| 4 | Receive | |
| 5 | Indication | |
| 6 | Signal Element Timing | |
| 7 | Byte Timing | |
| 8 | Signal Ground | |
| 9 | Transmit Data | |
| 10 | Control Signal | |
| 11 | Received Data | |
| 12 | Indication | |
| 13 | Signal Element Timing | |
| 14 | Byte Timing | |
| 15 | Protective Ground |
As the pinout describes, each bit and each byte are simultaneously signaled on the timing circuits (signal element timing, and byte timing) and transmitted on the data circuits. This allows the system to indicate clocking over the circuit, allowing the reciever to determine when to sample the line and retrieve a bit. When eight bits have been transmitted, the transmitter raises the byte timing wire. This acts as a double-check, making X.25 very reliable.
Since both sides have their own wires for all these functions, transmission works in both directions simultaneously and is thus, full-duplex.