The serial interface is a standard developed by the Electronic Industries Association (EIA). It defines the signals and voltages used when data is exchanged between a computer and a modem or serial printer.
The entire standard covers many more functions than are used in most data communications applications. Data is transmitted between the devices over a shielded serial cable with a 25-pin male (DB-25) connector to the modem and a 25-pin, 9-pin, 8-pin, or custom-built connector to the computer.
The use of a shielded cable when connecting a modem to a computer is recommended to ensure minimal interference with radio and television.
Pin assignments are factory-set in the modem to match the standard DB-25 assignments in the following table. DB-9 connectors for IBM/AT-compatible computers should be wired at the computer end of the cable as shown in the DB-9 column.
| DB-25 | DB-9 | Circuit | Function | Computer or Modem |
|---|---|---|---|---|
| 1 | - | AA | Chassis Ground | Both |
| 2 | 3 | BA | Transmitted Data | Computer |
| 3 | 2 | BB | Received Data | Modem |
| 4 | 7 | CA | Request to Send | Computer |
| 5 | 8 | CB | Clear to Send | Modem |
| 6 | 6 | CC | Data Set Ready | Modem |
| 7 | 5 | AB | Signal Ground | Both |
| 8 | 1 | CF | Carrier Detect | Modem |
| 12 | - | SCF | Speed Indicate | Modem |
| 20 | 4 | CD | Data Terminal Ready | Computer |
| 22 | 9 | CE | Ring Indicate | Modem |