Motor Drive Board - HP 9020 Service Manual

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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.