Initializing The - Radio Shack TRS-80 Service Manual

If
the printer
is
print
enabled
and
not busy,
the
input data
is
loaded
into
the
printer
by the data strobe from the input device.
The
printer
responds with an acknowledge pulse
to
acknowledge
reception
of
the data. Printable data, after
being inverted
by the input gating,
is
loaded
into
the
Random
Access
Memory
(RAM), consisting
of
two
4-bit
by
256
maximum
character
memories.
The address counter generates the seven address inputs
for
the
RAM.
Printable data
will
continue
to
be loaded
until
the
memory
is
filled
or
a
carriage return control
code
is
received by the printer.
In
either case, the printer
goes busy and
starts
the
print
cycle
by
activating
the forward carriage solenoid
(CIP).
This attaches
the carriage
to
the rotating
band,
moving the
carriage
from
left
to right
across the page.
The
address counter
is
clocked
(STROUT
(N)) to
address
the appropriate
location
in
memory
for
the character
to
be printed.
The
character
is
translated by
the character generator (ROM),
which
generates the impulse signals to drive
the head
solenoids.
At the
end
of
the printed
line,
the forward solenoid
is
deactivated
and
the reverse solenoid (CIR
(N)) is
activated
to
return the
carriage
to
the
left
margin.
The
printer
remains busy
until
the
end
of
the print cycle.
Also
at
the
end
of
the
line,
the paper
drive
solenoid
is
momentarily
activated,
causing the paper
to
advance
by
one
line.
When
the carriage
has returned
to
the
left
margin, the reverse solenoid
is
deactivated, the busy signal
goes away,
the
acknowledge
signal
is
generated,
and
the next
line
of data
is
taken
in.
The
left
margin indicator
(RTPSW)
signals the
logic
when
the carriage
reaches the
extreme
left
margin
of
the
printer.
As
the carriage
moves,
the oscillator
(ME1)
provides the control for print timing.
2.3
INITIALIZING
THE PRINTER
(Figure 2-2)
Before the
printer can accept data,
it
must
be
power primed, paper loaded
and
print enabled.
The power
prime operation
initializes
the printer logic to
a
ready
state.
The
print
enable operation,
resets
the
busy
line
to the interface
connector and
makes
the printer
ready to receive data.
2.3.1
POWER
PRIME
The power prime
signal
(PWRPRM
(N)) is
generated
when
the
power
switch
is
turned
on.
The
(PWRPRM
(N))
signal
gen-
erates the
(EOPPRM
(N))
signal
which
clears
the
print control logic.
2.3.2
PRINT
ENABLE
Before the printer
can
receive data,
it
must be enabled. This can be
done
by depressing the
PRINT
switch
on the operator's
panel.
2.3.3
PAPER
EMPTY
With paper loaded
in
the
printer
and
the printer
is
power primed and
print
enabled
the printer
can receive
data.
If
paper
is
not loaded
in
the
printer or a
paper
empty condition
exists
in
the
printer,
the paper
empty
switch opens, causing
a
BUSY
signal
to
the
input device.
A
paper empty
condition
is
detected
in
the printer by the
paper
empty
switch located
in
the path
of
the paper.
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PAPER EMPTY SWITCH
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LOGIC
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CONTROL
LOGIC
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Figure 2-2.
INITIALIZING
THE PRINTER
2-2