Epson PX-8 Technical Manual page 32

REV.-A
2.2.1.3
Power On/Off Timing
Power on/off has to be controlled by interrupting the sub-CPU 7508. Thus, either timing (the
POWER ON or RESET signal) will be a little delayed as shown in Fig. 2-5
POWERSW
(7508 POO)
POWER ON
(7508 P70)
OFF (7508 P71 )
(gate array OFF signal)
RESET
4
fls
~------i~--~----~
Fig. 2-5 Power On/Off Control Circuit
110ms
t: May vary depending on
operation in progress
when the POWER switch
is reset OFF.
The time delay sequence illustrated in Fig. 2-5 above is only a sample time sequence. The
power on/off operation permits any operation sequence in process, including the mechanical
operation of an I/O unit (e.g., the microcassette) currently in progress, to be completed and the
printer to be reinitialized before the power is off. The length of the illustrated time delay will vary
according to which mechanical and/or logic sequence must be completed. The off signal is used
to prevent a latch within the gate array.
2.2.1.4 Power On/Off Circuit Operations
Fig. 2-6 shows the circuit. When the POWER switch is set ON or RESET off, or an automatic
power on or off is input via software, the sub-CPU control program processes the power on or off
as an interrupt using port 70 as follows:
• Power on: P70 of the sub-CPU 7508 going low causes the anode of 015 to go low, turning pin
4 of IC '3E' high. This in turn causes the output at pin 14 of the next inverter, 120, to go low.
This signal is fed to transistors 06 and 023 through resistors R29 and 016 respectively, turn-
ing them on. This causes the VB (+) voltage to be output at the collector of 06, supplying the
operation voltage (logic circuit voltage: VL) to the elements on the board. The transistor 023
also supplies the VB(+) voltage to the battery-backed-up elements on the board. Thus, the
board is ready to operate. Port 42 of the 7508 sub-CPU controls the backup for the auxiliary
battery and can enable or disable conduction through transistor 020. When port 42 output is
high, the low level at pin 11 of IC 30 breaks down zener diode Z09, holding 020 in conduction.
In this way the LCD drive voltage is insured, the message, "CHARGE BATTERY", will be dis-
played whenever the main battery output voltage falls to or below the low voltage limit.
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