Theory
of
Operation 2-45
The logic
board
sends an initiating signal, RTEST,
and
controls the test through RAMPDN, SIGN,
and
VClO.
Monitor and Logic Circuits
These circuits control
and
monitor several functions.
lines
between this circuitry
and
the logic
board enable the microprocessor to monitor
head
voltage, backgate voltage
and
current,
head
temperature,
and
head
supply power transistor temperature. The logic board provides the go-down
signal
(GODOWN), used to shut down the printhead power supplies
on
the motor drive board.
If
the backgate voltage or the head voltage
is
out of specification (too high or too low), this circuitry
shuts down the power to the head.
Paper Sensor Board
The
paper
sensor board
is
mounted
on the printer to detect the presence
or
absence of paper. A
cable from the
paper
sensor board connects to
JS
on
the motor drive board. The
board
has
an
lED
and
a photo-conducting transistor. When
paper
is
present
and
interrupts the light flow from the
lED,
the transistor does not conduct,
and
the resulting signal from the motor drive board to the
logic
board
indicates
paper
present. The absence of
paper
causes the transistor to conduct
and
results
in
a signal indicating that no
paper
is
in
the printer or that the hole in the
paper
is
over the
sensor. This provides for registration of
paper
in
the printer
and
for form feed capability.
Printhead Interconnect Board
The
printhead
interconnect
board
is
an
integral
part
of the
printhead
assembly.
The
board
is
cabled
to
both
the
logic
board
and
the
motor
drive board. A single 22-pin
connector
provides
these
connections. A 16-pin section of
the
connector
is
cabled
to
motor
drive
board
connector
JPH,
providing
power
to
the
printhead. A 6-pin section
is
cabled to logic
board
connector
J2,
routing
data
and
control signals to
the
printhead.
Power
is
supplied by the printhead interconnect board to the
head
substrate
on
the printhead by a
head
voltage supply cable. Elastomeric connectors transmit data
and
control from the printhead
interconnect
board
to the chip modules where the signals are translated
and
provided to the
head
substrate via elastomeric connectors for dot burning.
Printer Operation
Integrated circuits
on
the chip modules contain a 560-bit shift register,
560
latches,
and
560
output
drivers (dot drivers). The printhead contains
560
resistors that, when heated, darken the thermal-
sensitive paper. The output drivers are open-drain field effect transistors (FETs). The resistors are
tied high to the
head
voltage from the motor drive board. Each resistor, nominally
100
ohms,
is
connected to a FET output driver.
Four lines from the logic
board
control the transfer of data to the printhead: NClEAR,
NDA
TA,
NSHIFT,
and
NBURN.
NClEAR
clears the shift register. NSHIFT shifts data
(NDATA)
one
bit
at
a
time into the shift register.
NBURN turns
on
the FETs (output or dot drivers) for the selected data
bits, causing the appropriate resistors to print
in
the 560-bit row. Another line,
head
data
out
(NHDOUT), returns the serial data from the printhead to the logic board for test purposes.
The
printer operates in either character
mode
or graphics mode. Character fonts are 7 bits wide by
12 bits high. Because the printhead
is
560
bits wide,
up
to
80
characters are printed across the
width of the head. Graphics data
is
printed in 8-bit groups, so up to 70 groups are printed at a time.
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