2-44
Theory
of
Operation
Motor Drive Board
The motor drive
board
performs several functions. It supplies power to the printhead, drives the
printer motor, measures printhead resistor values,
and
monitors paper status.
The motor drive board interconnects with the logic board via a
50-pin ribbon cable. The cable
interconnects J51
on
the motor drive board with
J4
on the logic board.
The printhead power cable
is
a IS-conductor cable that goes from connector
JPH
on
the motor
drive board to the printhead interconnect board
in
the printhead.
Connector JM provides the 6-conductor motor drive cable to the printer motor.
The
paper
sensor cable from the
paper
sensor board has four conductors
and
connects at
JS
on
the
motor drive board.
Head Power Supply and Control
The head power supply supplies the printhead with + 10 volts. This voltage
is
applied to the print
resistors to enable dot burning
on
the paper. This circuitry derives + 10 volts from the +
19
volts
provided to
it.
Backgate Supply
The backgate supply reduces - 12 volts to - 3 volts. It then provides the - 3 volts to the substrates
of the
NMOS driver chips.
+ 12V Shutdown
This circuitry shuts down the power to the printhead drivers
if
the head voltage or backgate supply
voltage goes
out
of specification. This averts damage to the printhead circuitry. When the printer
goes into idle state, this circuitry
is
also enabled. The idle state also reduces the motor current to
one-third of normal power,
and
shuts off the current to the paper sensor
LED.
Motor Drive, Control, and Shutdown
This circuitry provides voltage
and
control signals to the printer stepper motor. Five control signals
from the microprocessor
on
the logic board feed the motor control circuitry. Three signals select
one
of eight current drive levels to
be
applied to
one
of the four sets of motor windings chosen by
the other two signals. By applying these signals
in
the proper sequence with proper timing the
microprocessor accelerates, moves,
and
decelerates the motor.
When the printer
is
in
the idle state, a signal
is
asserted which causes the motor to draw minimal
current.
Paper
Sense
Circuit
This circuit provides a signal to the microprocessor
on
the logic board which indicates the presence
or absence of
paper
as detected by the paper sensor board. The paper sensor board
is
described in
a following section.
Printhead Resistor Measuring Circuit
The printhead resistor measuring circuit operates during power-up self-test. Each of the
560
print-
head
resistors which
is
in
range
is
measured for resistance value. This information
is
prOVided to the
microprocessor
on
the logic board which computes the average resistance
and
uses the result to
determine the burn time
and
the number of dots per burn.
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