Service manual
CIRCUIT
THEORY
UIC
U20
UIC
6
UIC
RESET
1
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3
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APU
Q21
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-..:- +5 V
-
GNO
Reset
Logic
Circuit
Theory
POWER ON
+5V
021
COLLECTOR
U1C5PIN
U1C6PIN
U1C4PIN
--------
APPROX.
350
ms
A
"low"
pulse applied
to
the
reset
interrupt
input,
pin 1
of
the
microprocessor
U7D,
causes
the
pro-
cessor
to
restart
execution
of
the
printer
operating
system
program stored in
the
ROM USD. A reset
pulse
occurs
when
the
power
is
turned
on.
An
external device connected
to
the
serial bus can also
generate a reset, initializing all devices connected on
the
bus
at
the
same
time.
A
short
time
after
the
power
is
turned
on,
the
5VDC
supply
rises
to
its
normal operating level. This
forward
biases
Q21,
allowing
C3
to
charge.
When
the
voltage across
C3
reaches 2
volts,
the
output
of
the
inverter U1C pin 6 goes
"low",
causing
the
output
of
the
inverter
U1C
pin 4
to
go
"high".
This
"low"
pulse
occurs
when
the
unit
is
switched
on. The reset
input
-
RES
- on
the
serial bus
(interface clear signal
-IFC
- on
the
4023
IEEE
interface)
forces
the
reset pulse
by
applying a
"Iow"
to
the
input
of
the
inverter U 1 C pin 1.
S