HP 750c Service Manual - Page 51

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 surface to the lineĆsensor. 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 the ASIC. The primary task of the processor is to configure the printĆcontrol ASIC and read data to control cartridge energy 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 pulse to a counter in the carriage microprocessor. 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 cartridge voltages the regulator will adjust the voltage to bring it back to the optimum value. Cartridge voltages are ON when printing a swath and briefly at powerĆup, but OFF when idle or accessing cartridges. PWM Signals The main board contains the drivers for the media and carriage motors. The drivers 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 drive the media up and out of the front of the plotter. The PB 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 drive the carriage toward the right side of the plotter. The PB signal to the carriageĆmotor drives the carriage towards the left side of the plotter. C4705-90000 Functional Overview 5-9