HP Compaq dc7100 DT Technical Reference Manual page 194

Keyboard
Keyswitch
Matrix
Figure C-1. Keystroke Processing Elements, Block Diagram
When the system is turned on, the keyboard processor generates a Power-On Reset (POR) signal
after a period of 150 ms to 2 seconds. The keyboard undergoes a Basic Assurance Test (BAT)
that checks for shorted keys and basic operation of the keyboard processor. The BAT takes from
300 to 500 ms to complete.
If the keyboard fails the BAT, an error code is sent to the CPU and the keyboard is disabled until
an input command is received. After successful completion of the POR and BAT, a completion
code (AAh) is sent to the CPU and the scanning process begins.
The keyboard processor includes a 16-byte FIFO buffer for holding scan codes until the system is
ready to receive them. Response and typematic codes are not buffered. If the buffer is full (16
bytes held) a 17th byte of a successive scan code results in an overrun condition and the overrun
code replaces the scan code byte and any additional scan code data (and the respective key
strokes) are lost. Multi-byte sequences must fit entirely into the buffer before the respective
keystroke can be registered.
C-2
Num
Lock
Matrix
Drivers
Keyboard
Matrix
Processor
Receivers
361834-002
Scroll
Caps
Lock
Lock
Data/
CLK
Technical Reference Guide
Keyboard
Interface
(System Unit)