The following table describes the X.25 coprocessor 37-pin connector pin assignments.
| X.25 Coprocessor 37-Pin Connector Pin Assignments | |||||
| Pin | Circuit Description | Symbol | X.21bis /V24 | X.21bis /V35 | X.21 |
| 1 | Reserved | ||||
| 2 | Transmitted data | TXD | x | ||
| 3 | Received data | RXD | x | ||
| 4 | Request to send | RTS | x | x | |
| 5 | Clear to send | CTS | x | x | |
| 6 | Data set ready | DSR | x | x | |
| 7 | Signal ground | GND | x | x | x |
| 8 | Carrier detect | CD | x | x | |
| 9 | Cable ID 0 | ID0 | x | x | |
| 10 | Transmitted data (A) | T (A) | x | ||
| 11 | Control (A) | C (A) | x | ||
| 12 | Received data (A) | R (A) | x | ||
| 13 | Indication (A) | I (A) | x | ||
| 14 | Transmit clock (A) | S (A) | x | ||
| 15 | Cable ID 1 | ID1 | x | x | |
| 16 | Receive clock (B) | RX CLK (B) | x | ||
| 17 | Transmitted data (B) | TXD (B) | x | ||
| 18 | Transmit clock (B) | TX CLK (B) | x | ||
| 19 | Receive data (B) | RXD (B) | x | ||
| 20 | Data terminal ready | DTR | x | x | |
| 21 | Remote loopback test | RLBT | x | ||
| 22 | Call indicate | CI | x | x | |
| 23 | Reserved | ||||
| 24 | Transmit clock | TX CLK | x | ||
| 25 | Test indicate | TI | x | ||
| 26 | Receive clock | RX CLK | x | ||
| 27 | Local loopback test | LLBT | |||
| 28 | Transmitted data (B) | T (B) | x | ||
| 29 | Control (B) | C (B) | x | ||
| 30 | Received data (B) | R (B) | x | ||
| 31 | Indication (B) | I (B) | x | ||
| 32 | Transmit clock (B) | S (B) | x | ||
| 33 | Reserved | ||||
| 34 | Receive clock (A) | RX CLK (A) | x | ||
| 35 | Transmitted data (A) | TXD (A) | x | ||
| 36 | Transmit clock (A) | RX CLK (A) | x | ||
| 37 | Received data (A) | RXD (A) | x | ||
| NOTES: | |||||
| (a) and (b) indicate pins that are associated to form pairs. | |||||