HP 750 - Manual (Page 51)

Functional Overview C4705-90000 5-9
LineĆSensor PCA
The lineĆsensor PCA is used to calibrate the writing system for mechanical tolerances as well
as sensing the left and right edges of the media. It consists of a lightĆsensing IC connected to
the PCA through a ZIF connector. The resulting voltages are coupled and then measured by
an eightĆbit, analogĆtoĆdigital (A/D) converter whose bits are sent serially to the carriage
microprocessor. A green and blue LED illuminates the surface being read by the lineĆsensor.
A dual lens system with a unity conjugate ratio system directs the light from the illuminated
surfaceto thelineĆsensor.
TemperatureĆSensor IC
A temperatureĆsensor IC is used to measure ambient temperature in the PCA near the print
heads. It is used to calibrate the cartridge temperature control circuitry.
LinearĆEncoder IC
A linear encoder detects cartridge carriage position, as the carriage assembly moves along the
encoder strip. The linearĆencoder IC is connected to the carriage PCA via a 5Ćpin ZIF
connector.
Print Control
The carriage processor operates at 12.288 MHz. The clock signal is generated by a
crystalĆoscillator circuit, internal to theASIC. Theprimary task of theprocessor is to
configuretheprintĆcontrol ASIC and read data to control cartridgeenergy and cartridge
calibration. It communicates with the servo processor on the main PCA using a serial port.
The print control (carriage) ASIC receives pixel data from the processorĆsupport ASIC on the
main PCA via four synchronous serial channels and an onĆboard line driver. Signals from the
printĆcontrol ASIC fire the nozzles in the print heads. Whenever a print head nozzle is fired,
a comparator sends a pulseto a counter in thecarriagemicroprocessor.
The dualĆvoltage, remote sensing circuitry sends an analog sensing signal to the dual voltage
regulators on the main PCA. When the voltage to the print heads deviates from the optimum
cartridgevoltages theregulator will adjust thevoltageto bring it back to theoptimum value.
Cartridge voltages are ON when printing a swath and briefly at powerĆup, but OFF when idle
or accessing cartridges.
PWM Signals
Themain board contains thedrivers for themedia and carriagemotors. Thedrivers are
controlled by the PA and PB signals from the processorĆsupport ASIC. PA and PB are
pulseĆwidthĆmodulated (PWM) signals. The PA signal to the mediaĆmotor driver causes the
mediaĆmotor to drivethemedia up and out of thefront of theplotter. ThePB signal to the
media motor driver causes the media motor to drive the media down, over the entry platen
and out of the plotter. The PA signal to the carriageĆmotor driver causes the carriage motor to
drivethecarriagetoward theright sideof theplotter. ThePB signal to thecarriageĆmotor
drives thecarriagetowards theleft sideof theplotter.