User`s guide

Scan Code

Processing

Note: The infonnation that follows gives an

overview of how scan codes are

generated at the keyboard. Refer to the

^propriate IBM Technical Reference

Manual for detailed information on

these topics.

Undemeath the keyboard is a grid of electrical circuitry.

When a key is pressed, contact is made with the grid,

creating a closed circuit. The keyboard microprocessor

uses electrical current to constantly scan this grid, even

when the keyboard is not in use. Once a closed circuit is

detected, the microprocessor encodes the position of the

keypress, translates the keypress to an IBM scan code,

and transmits this scan code to the host computer. A scan

code,

sometimes called a "key code", is a numerical

value that uniquely identifies each keyboard key.

Each key or keyswitch on the computer is represented by

two scan codes: a "down" scan code (make code), sent

when a key is pressed, and an "up" scan code {break

code),

sent when the key is released. For example, on the

IBM PC/XT and PS/2™ Models 25 and 30, keyswitch

number

"1",

the Escape key, produces scan code 01

hexadecimal (hex) on make and scan code 81 hex on

break. Break codes are formed by adding 80 hex to the

make code.

The IBM AT and PS/2 Models 50,60, and 80 use a

different set of scan codes than the PC/XT. For example,

keyswitch number

"1"

on the AT produces scan code 76

hex on make and scan code

76 hex on break. AT

break codes are formed by preceding the make code with

hex.

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