Radio Shack TRS-80 Service Manual page 13

2.6
CARRIAGE
AND
RIBBON
MOVEMENT
(Figure 2-5)
Mounted
on
the carriage are the
print
head, ribbon
cartridge
and
forward/reverse
carriage solenoids. The
carriage
moves
from
left
to right during the
print operation
and from
right to
left
when
returning
to
the
start position.
The
ribbon
is
auto-
matically advanced, during the forward
and
reverse
movement
of the
carriage,
making
full
use
of
the
ribbon.
All
carriage
movement
is
a result of the
drive signals
from
the electronics board,
causing the forward and
reverse solenoids
to seize
the band.
2.6.1
LEFT
AND
RIGHT MARGIN
INDICATORS
Located on
the
left
side
of
the
printer,
is
a light emitting diode (LED) and
phototransistor device which
indicates
if
the
print
head
is in
the
start position.
If
the carriage
is
sitting at the
left
margin, the carriage arm
extends
into
the
slot
in
the
device,
preventing the LED
from
energizing the phototransistor. This
generates
a
low ready
to print (RTP) signal
to
the
print control
logic.
When
the carriage
is
to
the
right of the
indicator,
the arm
is
out
of
the
slot,
permitting
light to
energize the photo-
transistor
and
thus
make RTPSW(N)
high. Therefore, the
RTPSW(N)
is
low
when
the carriage arm
is
in
the
indicator
and
high
when
the
arm
is
out
of
the
indicator.
The
right
margin
indicator,
located
on
the
right side
of
the
printer,
is
a
LED
and
phototransistor device
identical to the
left
margin
indicator.
When
the carriage reaches
the end
of travel
at
the extreme
right margin, the
end
of print
(EOPSW)
signal
is
generated, which
in
turn generates
EOPSW(N)
and
is
sent to the
print control
logic.
The EOPSW(N)
signal
is
low when
the
carriage
arm
is
in
the indicator and
high
when
the carriage arm
is
out
of the
indicator.
2.6.2
CARRIAGE
DRIVE
The
carriage
is
moved
by the
drive motor,
rotating fibre
band and
forward/reverse
solenoids.
The
drive motor
rotates
at a
constant speed
rotating the
fibre
band
clockwise.
If
the forward
solenoid
(CIP
(N)), is
energized, the solenoid
seizes the top
half of
the band moving
the carriage
to
the
right.
Energizing the reverse
solenoid, (CIR
(N)),
attaches the carriage
to the
bottom
half of
the band, moving
the carriage
to
the
left.
2.6.3
CARRIAGE
DRIVE SIGNALS
The
signals
for
energizing the
forward/reverse
solenoids are
generated
by
flipflop
ME3.
When
the printer
is
ready
to print
the received
line of data, the
flip
flop activates the forward
drive signal CIP
(N).
The
signal
is
amplified by the
power
driver,
the output
of
which
activates the forward
solenoid.
The
forward
solenoid remains
active
until
a carriage return code
is
detected
in
the data
at
the
end
of a
line or
until
the buffer
full
condition occurs
at
the
end
of a
full
line of data. Either
of
these
two
conditions causes
the
flip
flop to deactivate the
forward
solenoid
and
activate the reverse solenoid
signal CIR(N)
to
return
the carriage
to
the
left
margin.CIR
(N)
is
deactivated
when
the RTP
switch
is
reached.
2.6.4
RIBBON
DRIVE
The mobius
loop ribbon
is
advanced
during the forward and
reverse
movement
of the
carriage.
The
ribbon
drive shaft
rotates clockwise feeding the ribbon out
of
the back
of
the
cartridge,
around
to
the
front of the
print
head and then back
into
the
front of the
cartridge.
Movement
of the ribbon
drive shaft
is
the
result of the
two one-way
slip clutches,
one
for
forward
movement
and one
for
reverse
movement. Two
ribbon
drive wires, upper and
lower, are
attached
to
the
left
and
right
sides
of the printer and looped
one-half turn around
the upper and
lower
clutch.
The
drive wires
are
arranged such
that
when
the
carriage
moves
to the
right,
the top pulley
is
engaged and when
moving
to the
left,
the
bottom
pulley
is
engaged.
II5VAC
FROM
TRANSFORMER
—-0-0
—
HZh-0-
Figure 2-5.
CARRIAGE
AND
RIBBON
MOVEMENT
2-5