Service manual
CIRCUIT
THEORY
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Top
plate
.print Wire Drive Circuits
U5D
outputs
eight
signals
NDO
- ND7 (ND = Needle Drive)
from
parallel
port
B
(PBO
- PB7). These
signals are inverted
by
U5B
and U6B. The
output
of
the
inverters
control
current
amplifier
transistors
Q1
-
Q8.
A
print
wire
(needle) is fired
when
an
output
transistor
is
forward
biased,
allowing
current
to
flow
through
the
coil
from
the
26VDC
supply.
CR
1 -
CR
12
suppress CEMF developed
by
the
coils.
The inverters U5B and U6B are
7406
open collector Hex/Inverters. A one
shot
circuit generates + VCC
for
U6B
and
U5B
for
500
microseconds
when
a
print
wire
is fired. This
protects
the
print
wire
coils
by
limiting
the
current
through
the
coils. See Coil Protection
Circuit
and
Timing
on page
14.
13