User manual

TECHNICAL REFERENCE
8-23
The Serial Interface
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.
FCC regulations require the use of a shielded cable when connecting a modem to a computer to ensure minimal
interference with radio and television.
Pin assignments are factory-set in the U.S. Robotics 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.
Serial Interface Pin Definitions
Signal Source
DB-25 DB-9 Circuit Function Computer/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