Keyboard - Epson PX-8 Technical Manual

REV.-A
2.6.1 Resetting By SW1 (POWER) and SW2 (RESET)
To use either switch SW1 (POWER) or SW2 (RESET or the RS signal on the circuit dia-
gram), it is requisite that the sub-CPU 7508 be operating normally. When either switch is
pressed, the sub-CPU accepts the signal as an interrupt and turns its port 40 low. This signal is
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fed to the RST terminal of the gate array, GAH40D (6A), and is output from the RSO terminal, re-
setting the main CPU and the slave CPU, etc.
2.6.2 Resetting By SW5 (INITIAL RESET)
Switch SW5 allows the sub-CPU itself to be reset. It initializes the sub-CPU's internal
settings, including the built-in calender clock setting, etc. This is the only difference between the
reset from switchs and the reset by switches SW1 and SW2.
2.6.3 OFF Signal
The OFF signal is provided in association with computer resetting in order to prevent inconven-
iences such as latch of flipflops in the gate arrays, etc. It provides the gate arrays with the follow-
ing functions:
<GAH40D>
Supplies the reset signal to Z80 and 6303, etc.
Disables the Chip Select signal to the IPL ROM.
Disables the Z80 read signal line.
Disables the interrupt signal line to Z80.
Resets all FFs.
Disables all outputs other than the above.
<GAH40M>
Prevents latching of the gate array FFs by forcing the outputs in the high impedance state.
2.7 Keyboard
The keyboard has 72 keys (including function key switches), three light emitting diodes (LEDs), 23
diodes, and three resistors for the LEDs mounted on it. The keyboard is structured in a matrix
which is scanned by the sub-CPU 7508. Keyed in data are sent to the main CPU Z80 from the
cub-CPU via the gate array GAH40M. In preparation for a situation where the normal keyed-in
data transfer to the main CPU is hampered, the sub-CPU incorporates a 7 -character key data
buffer. Fig. 2-64 is a block diagram illustrating the keyboard input operation.
2-64