Hardware Reference
In-Depth Information
to the motherboard a serial packet of data containing the scan code for the key that was
pressed.
This is called the
Make code
. When the key is released, a corresponding
Break code
is
sent,indicatingtothemotherboardthatthekeyhasbeenreleased.TheBreakcodeisequi-
valent to the Make scan code plus 80h. For example, if the Make scan code for the A key
is 1Eh, the Break code would be 9Eh. By using both Make and Break scan codes, the sys-
tem can determine whether a particular key has been held down and determine whether
multiple keys are being pressed.
In motherboards that use an 8042-type keyboard controller, the 8042 chip translates the
actual keyboard scan codes into one of up to three sets of system scan codes, which are
sent to the main processor. It can be useful in some cases to know what these scan codes
are, especially when you're troubleshooting keyboard problems or when reading the key-
board or system scan codes directly in software.
When a keyswitch on the keyboard sticks or otherwise fails, the Make scan code of the
failed keyswitch usually is reported by diagnostics software, including the power-on self
test(POST),aswellasconventionaldisk-baseddiagnostics.Thismeansyoumustidentify
the malfunctioning key by its scan code. See the Technical Reference section of the DVD
included with this topic for a comprehensive listing of keyboard key numbers and scan
codes for industry-standard 101/102-key (Enhanced) and 104-key Windows keyboards.
Bylookingupthereportedscancodeonthesecharts,youcandetermine whichkeyswitch
is defective or needs to be cleaned.
Note
The 101-key Enhanced keyboards are capable of three scan code sets. Set 1 is the default.
Some systems, including some of the IBM PS/2 machines, use one of the other scan code
sets during the POST. For example, the PS/2 P75 uses Scan Code Set 2 during the POST but
switchestoSet1duringnormaloperation.Thisisrare,butononeoccasionitreallythrewme
off in diagnosing a stuck key problem. It is useful to know whether you are having difficulty
interpreting the scan code number, however.
IBM originally assigned each key a unique key number to distinguish it from the others.
This is helpful when you are trying to identify keys on foreign keyboards that might use
symbols or characters different from what the U.S. models do. In the Enhanced keyboard,
most foreign models are missing one of the keys (key 29) found on the U.S. version and
have two additional keys (keys 42 and 45). This accounts for the 102-key total instead of
the 101 keys found on the U.S. version.
