Epson PX-8 Technical Manual page 89

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Fig. 2-64 Keyboard Input Operation Block Diagram
The keyboard keys are scanned by the sub-CPU which controls the key data outputs as follows:
(1) Key switch (CTRL, SHIFT(R), SHIFT(L), CAPS LOCK, NUM GRPH, and CTRL (right of the space-
bar) output control
When any of these keys is pressed for the first time (i.e., it is stet), the sub-CPU issues a
MAKE code. When it is pressed for the second time (i.e., it is released), the sub-CPU issues a
BREAK code. This is required for the correct keyboard data input, because the keyboard input
mode is controlled by these keys and the main CPU has to be informed whether anyone of
them is in effect. Fig. 2-65 illustrates a sample sequence of key strokes, which includes shift
operations in the alphanumeric mode: The sub-CPU issues the MAKE and BREAK codes for
the left SHFT key when it is locked and released respectively. The main CPU can display the
intended upper case and lower case alphabets as shown below.
Character data keys
Function key
(left SHIFT)
Key codes output
from 7508
Characters displayed
by main CPU
Fig. 2-65 Keyboard Data Input And Mode Control Sample
2-65