Epson PX-8 Technical Manual page 92

REV.-A
2.7.2 Key Signal Input
The keyboard has a 9 x 8 matrix structure which uses nine keyboard scan (KSC) signals and eight
keyboard return (KRTN) signals. The scan signals are used as a reference and a pressed key is
identified by examining the corresponding return signal lines for make.
Key input is controlled by the 4-bit sub CPU 7508 which is programmed to read, when the com-
puter is initialized, the setting of the DIP SW2 assembly, as well as the key switches. The read key
data and the SW2 setting are bit-serially transferred to the IC 4C (GAH40M), where the data are
converted from serial to parallel, and further transferred to the main CPU. Fig. 2-68 is a block dia-
gram of the keyboard matrix.
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A
~
1
1
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7
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B
1 1
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5
I 1
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KRTN 0 ~ 7
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MAPLE board
Keyboard
...
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CN4
Fig. 2-68 Keyboard Matrix
Normally, IC 1 E maintains all the KSC signal lines at the low level. Pressing a key causes a current
to flow from the corresponding KRTN signal line on the MAPLE board to the IC through the
diodes, pulling the KRTN signal line low. 7508 can detect that a key is pressed from the level
change of the KRTN signal line. However, it cannot identify the pressed key which turned the line
low. To accomplish this identification, the 7508 monitors the KRTN signal line while outputting
pulses over the KSC signal lines as shown in Fig. 2-69. By examining both the KRTN signal line
turning low and the KSC signal line, over which a negative going pulse is currently output, the
7508 is able to identify which key has been depressed.
2-68