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Printed circuit assembly (pca) overview – HP 750 User Manual

Page 49

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Functional Overview

C4705-90000

5-7

Media Sensor

The media sensor is an optical sensor and is mounted towards the right end ofthe entry

platen.
Two articulated levers block the optical path between the emitter and the receiver when

media is not loaded. When media is loaded, both levers are rotated, thus clearing the optical

path and changing the sensor state from open to closed. This doubleĆlever system provides

the plotter with leading and trailing edgeĆsensing capability (for mediaĆlength measurement).

The media sensor is electrically connected to the main PCA.

Drop Sensor

The drop sensor is an infrared detector mounted in the service station. It is used to test

whether the nozzles in the cartridges are operating properly. It is part ofa dropĆdetection

system that measures the time from the firing of a nozzle to the sensing of a drop (typically

< 900

m

sec). The operation ofthe sensor system depends upon an illuminated infrared

optical path, in which the received power is modulated a small amount by a drop passing

through an aperture on the way to the waste containment system. The illumination is

provided by an infrared LED, which is regulated by an automatic power control to provide a

fixed operating point in the optical receiver photoĆdiode. The automatic power control

compensates for contamination, device aging and environmental effects.

PrimerĆCam Sensor

An optical sensor mounted on the primer assembly determines the initial position ofthe

primer assembly cam. When the stepper motor that drives the primer assembly rotates in one

direction, it obtains the values for selecting each cartridge for priming. When rotated in the

opposite direction, a clutch engages a diaphragm to provide the vacuum for priming.

Printed Circuit Assembly (PCA) Overview

Main PCA

There are two clocks on the main PCA. The main clock provides a 32.00ĆMHz signal directly

to the main processor. The 32.00 MHz are divided by the main processor, and the resulting

signal is provided to the processorĆsupport ASIC and the BiĆtronics gate array. A separate

clock provides a 12ĆMHz signal to the servo processor.
To perform its functions, the main processor must have access to its memories (ROM and

DRAM). To create plots at the direction ofan external controller, it must be able to

communicate with the controller by its interfaces (MIO, BiĆtronics and RSĆ232ĆC). To address

the ROM or DRAM interface, the main processor places the appropriate address on the

address bus and reads or writes the data directly. Data is passed to the main processor by a

transceiver in the case ofROM or DRAM SIMM data. MIO or BiĆtronics input and output

data passes through another transceiver on its way to the main processor.