Master Series Service Manual
5.3 Internal I/O
Slow peripherals:- These are subsystems which are provided with data
from port A of the system VIA. This data is stable until next
programmed by the
CPU.
Keyboard
General description:- 93 keys are provided. 92 of these are in a
modified 8 x 13 matrix as shown in figure 5. A keyboard encoder, IC16
is used to scan the keyboard. During idle (free run) mode, pressing
any key will cause an IRQ to be generated via the system 6522. A
connection is provided from IC16 to a 6522 'CA' type connection. Hence
the interrupts thus generated are controlled by the 6522 control
register. Depression of either of the shift keys, or the control key
does not cause an interrupt to occur.
Keys are arranged as a QWERTY style keyboard with extra keys for a
numeric keypad. Ten additional 'function keys' together with cursor
control buttons, etc., are provided.
The "BREAK" key will reset the CPU and abort any access to the
clock/RAM chip. To prevent accidental operation, a mechanical lock is
provided. This is a plastic cam which is rotated through 90 degrees to
stop the keytop from being depressed.
Keyboard Operation:- During free run mode, the keyboard column lines
are continually scanned by incrementing a counter, decoding its
outputs and pulling low a column line. Any key depressed will cause
the interrupt to be generated. A signal, KeyBoard ENable is generated
to stop free running mode. The counter contents are now loaded by CPU
operation to determine on which row the key was pressed. The rows are
then individually selected to
determine which key was pressed.
IC16 is supplied with data from the slow data bus:-
PAO to PA6:- These are the slow bus connections. PAO to PA3 are the
column select inputs and PA4 to PA6 are the row select inputs. PA7 is
a three state connection which is driven active low when a row/column
combination describes a depressed key.
RO to R7:- The keyboard row input connections are normally held high
by internal pull-up resistors. If a key is depressed it will cause the
appropriate row connection to be pulled low when its column is
selected.
CO to C14:- These open collector column driving outputs are
sequentially taken active low in auto scan mode at a rate of 1MHz. In
polled mode (nKBEN active low) the slow bus inputs PAO to PA3
determine which output will be low. The selected column output is a
direct decode of these inputs.
CA2:- Connected to the system VIA, this output will cause the VIA to
generate an nIRQ. The line will be active low when an active key is
detected.
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