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PC Hardware in a Nutshell, 3rd Edition by Barbara Fritchman Thompson, Robert Bruce Thompson

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DB9 pin definitions

When IBM introduced the PC/AT in 1984, it used a nonstandard DB9M 9-pin serial connector that included only the commonly used pins. Because this connector is physically smaller, it allowed putting both a serial port and a DB25F parallel port on the same expansion card bracket. The market presence of IBM and the passage of years have combined to make this DB9M serial connector a de facto standard. Like the DB25, DB9 connectors are available in male and female versions, designated DB9M and DB9F, respectively. By convention, DTE devices use the DB9M. Few DCE devices use DB9 connectors. However, those that do usually use a DB9F. Figure 22-2 shows a DB9 connector.

A DB9 connector

Figure 22-2. A DB9 connector

Although its pinouts don’t map to either DTE or DCE, the DB9M serial connector is considered an “honorary” DTE device, with DTE-to-DCE interfacing accommodated by the cable. The DB9 connector includes the nine important signals, but the pinouts differ from DB25, as Table 22-2 shows.

Table 22-2. DB9 serial port pin assignments

Pin

I/O

CCITT

EIA

RS

Common name(s)

Abbreviations

1

In

109

CF

CF

Data Carrier Detect, Carrier Detect, Receive Line Signal Detect

DCD, CD, RLSD

2

In

104

BB

BB

Receive Data

RD, RxD, SIN

3

Out

103

BA

BA

Transmit Data

TD, TxD, SOUT

4

Out

108.2

CD

CD

Data Terminal Ready

DTR

5

-

102

AB

AB

Signal Ground

SG

6

In

107

CC

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