PC Hardware in a Nutshell, 3rd Edition

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.

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