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Epson PX-8 - 3.3 Starting To Use CP;M

Epson PX-8
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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

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