UPGRADING REPAIRING PCs

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Table 6.2 25-Pin (PC, XT, and PS/2) Serial Port Connector
Continued
Pin Signal Description I/O
8 CD Carrier detect In
9 — +Transmit current loop return Out
11 — -Transmit current loop data Out
18 — +Receive current loop data In
20 DTR Data terminal ready Out
22 RI Ring indicator In
25 — -Receive current loop return In
Table 6.3 9-Pin-to-25-Pin Serial Cable Adapter Connections
9-Pin 25-Pin Signal
1 8 CD Carrier detect
2 3 RD Receive data
3 2 TD Transmit data
4 20 DTR Data terminal ready
5 7 SG Signal ground
6 6 DSR Data set ready
7 4 RTS Request to send
8 5 CTS Clear to send
9 22 RI Ring indicator
Note
Chapter 6—Serial Ports and Modems
Macintosh systems use a similar serial interface, defined as RS-
422. Most external modems in use today can interface with
either RS-232 or RS-422, but it is safest to make sure that the
external modem you get for your PC is designed for a PC, not a
Macintosh.
Current Loop Serial Devices and 25-Pin Serial Ports
Whereas normal RS-232 serial devices can be connected to either a
9-pin or a 25-pin serial port, and 9-pin devices can be adapted to a
25-pin port, another type of serial device known as a current loop
device will work with 25-pin ports only.
By comparing Figures 6.1 and 6.2, you can see that the currentloop
pins (9, 11, 18, and 25) have no corresponding pins in the
9-pin serial port. Thus, if you need to connect current-loop devices