Wang 5536 - Page 21

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2.3.1.6 Keyboard Interface (Figure 2-4)
The keyboard interface accepts characters from the keyboard and generates a
processor interrupt, enabling data to be transferred by the CPU to CRT
memory and thus to the display. During each transfer, keyboard interface
logic supplies the keyboard with a signal to inhibit additional transfers
until the current character has been accepted by the processor.
The Z80A CPU responds to keyboard interrupts by issuing a command which
forces a hard-wired signal, C7, onto the data bus. The CPU interprets C7
as a RESTART command.
The keyboard inter face also performs these functions:
o Keystroke debounce
o Repeat keys
2.3.1.7 Keystroke Processing
There are two types of keystroke operations: single key and repeat key.
Single keystroke processing requires the keyboard interface to:
o Recognize the keystroke
o Inhibit the generation of additional keystrokes
o Inhibit the recognition of additional keystrokes
o Provide time (lOms) for keystroke debounce
o Generate an interrupt request
o Clear interru:Jt and character/keystroke inhibits as soon as the CPU
reads the character.
Repeat keystroke processing requires these additional interface functions:
o Initiate a quarter-second delay to recognize a request for repeat
o Recognize repeat keys
o Generate repetitive interrupts every 60 ms during normal horizontal
movement
0
0
Shorten the repeat delay to, 30 ms during horizontal
(The cursor crosses more character spaces when moving
when moving the same distance vertically.)
Permit any key to be repeated when the desired key
simultaneously with any repeat key.
2.3.1.8 IN-OUT Decoders (Figure 2-5)
cursor movement.
horizontally than
is pressed nearly
Certain memory instructions instruct the CPU to transfer data either into
or out of an I/0 device rather than Workstation memory. These instructions
are designated IN and OUT, followed by two-digit hex identifiers. When the
CPU receives an IN or OUT instruction, the IN-OUT decoders identify the
device. For example, a signal from the IN decoder (during an appropriate
IN instruction) causes the keyboard interface to place a character code
onto the data bus. The instruction sequence causes the CPU to examine the
character and store it in the CRT memory buffer, to be displayed on the CRT
display.
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