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Epson PX-8 - Page 32

Epson PX-8
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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.
2-8

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