Keyboard And Input Device Scanning - HP 9020 Service Manual

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Theory of Operation
The keyboard service request latch is set when the keyboard microprocessor outputs a keycode to
the output register. Then, on the next polling cycle, bit 6 on the lOP bus is set, signifying to the
processor that a peripheral on SC6 is requesting service.
Two 8-bit bus test latches have their inputs and outputs tied to the bus. At power-up, operating
system code tests data path integrity by writing data into and then reading the data from these
latches via RI5.
Keyboard and Input Device Scanning
The keyswitch board scans 15 columns of keys with 8 keys per column. It interconnects with the
keyboard electronics board via a 20-pin ribbon cable and the electronics board bus connector
(KBOC).
The keyboard microprocessor, which resides on the keyboard electronics board, has 1 K bytes of
ROM and 64 bytes of RAM. The microprocessor constantly scans, or interrogates, the keyswitch
board. It sends the 4-bit column address to the keyswitch board. That board responds by sending
the status of all 8 keys of the specified column to the microprocessor, which processes the data. It
then sends the next column address, and the process continues.
When a new key is pressed, the microprocessor translates the address of the key into a 7 -bit
keycode which it then shifts into a holding register along with two status bits: Shift and Control. A
service request signal is then sent to the lOP bus. The lOP responds by reading RIO. Lockout logic
prevents overwriting of the keycode in the holding register for a maximum of 2.25 ms, enabling it to
be read by the lOP.
The keyboard microprocessor controls the two LEOs on the keyboard and the initial delay and
repeat rate of keys which are held down. When a key is pressed on the keyboard which requires an
LEO change, that keycode is sent to the lOP which in turn responds by updating all LEOs and the
auto-repeat rate by writing to RIO.
The auto-repeat rate determines the initial delay and repeat interval of keys that are continuously
held down. The default is 500-millisecond delay and 20-millisecond repeat interval. Another 30
combinations can be selected.
The desired keyboard language is determined by the insertion/deletion of four resistors on the
keyboard electronics board. This I-of -16 language code is translated by the operating system to
select the predetermined language. A replacement keyboard electronics board is configured for
ASCII and has all resistors in place. Refer to Chapter 4 for the language/resistor combinations and
Chapter 5 for language/keycap configurations.
Buffers on the processor board provide isolation for signals from the optional input device. The
input device plugs into connector TB located at the right front of the computer, under the keyboard.