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

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DB25 pin definitions

EIA-232D defines pin assignments, but does not specify physical connectors. In the PC environment at least, a well-established de facto standard does exist for 25-pin serial connections. By convention, DTE devices (e.g., PC serial ports) use a 25-pin male D-sub plug connector, designated DB25M. DCE devices (e.g., modems) use a 25-hole DB25F female socket connector. There are exceptions. For example, some serial printers are DTE devices but use a DB25F connector. Figure 22-1 shows a DB25 connector.

A DB9 connector

Figure 22-1. A DB9 connector

Table 22-1 lists DB25 pin assignments. The column headings are self-explanatory, except the following:

I/O

Signal direction, relative to the DTE. For example, a DCE (modem) asserts voltage onPin 5 (CTS) to notify the DTE (serial port) that it is ready to accept data from the serial port. We list this signal direction (from the DCE to the DTE) as In. Pins 1 and 7 are ground pins, and have no signal direction. Pin 11 is unassigned, and so has no signal direction.

CCITT, EIA, and RS

The circuit numbers/names used by CCITT, EIA, and RS standards documents, respectively. Circuits with a hyphen (-) are defined by the standard in question but not assigned a name. Circuits with a blank box are neither defined nor named.

Table 22-1. DB25 serial port pin assignments

Pin

I/O

CCITT

EIA

RS

Common name(s)

Abbreviations

1

-

101

-

AA

Chassis Ground ...

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