HP 750 - Manual (Page 49)

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.