Epson PX-8 Technical Manual page 43

REV.-A
2.2.3.3 Circuit Operation After Low Voltage Detection
The 4-bit sub-CPU raises its port 42 (pin 37) high and controls the battery switching from main to
auxiliary battery power as follows:
When power is on
• Since port 70 (POW ON) of the sub-CPU is low when power is on, the output at pin 4 of IC 3E is
inverted high, holding pin 10 of the IC 3D low as inverted by the IC. The resistor R98 (100
kohms) and the zener diode ZD8 (4V) are connected in series across this pin and the anode of
the auxiliary battery.
Normally, the auxiliary battery is fully charged, since it is always charged in the trickle charge
mode, and has an output voltage of 4V or above. Thus, the zener diode intermittently breaks
down. This in turn causes the transistors 015 and 03 to alternate conduction and cut-off. This
operation intermittently continues until the auxiliary battery voltage reaches 4V (discharge final
voltage). When 03 starts conduction after the discharge final voltage is reached, a current
flows from the auxiliary battery, which is connected to the emitter of 03, to the collector; i.e., to
the VB line, supplementing its power which is being supplied from the main battery. This oper-
ation ensures that the computer operation, such as microcassette rewind, etc., which is in pro-
gress when low voltage is detected, is normally completed.
• The high output from port 42 of the sub-CPU is also fed to the base of transistor 020 to enable
the backup voltage supply to the VB+ line from the auxiliary battery. The backup voltage supply
ensures that the computer will continue to operate until an operation termination sequence is
executed and the low voltage condition is detected. Subsequently, the "CHARGE BATTERY"
message is displayed on the LCD panel.
When power is off
• When power is off, the high level of port 70 of the sub-CPU (POW ON) holds the output at pin 4
of the IC 3E low, disabling the AND logic consisting of pins 8, 9, and 10 of the IC 3D and hold-
ing the output at pin 10 high. This disables the power supply from the auxiliary battery to the
VB line. However, the power supply to the backup line through the transistor 023 is enabled.
2-19