Lexmark W820 Service Manual page 44

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Output to motors, solenoids, and other components
After comparing input values with timing and reference values, the MCU makes a
decision on the current status of the printer and responds appropriately. The MCU
may switch on or off motors, solenoids, or other components. The MCU may also
signal that an error occurred.
Example follows:
At the start of a print cycle, the MCU switches on the Main Drive Motor, and then the
Feed Clutch. The MCU uses the Feed Clutch actuation as a timing marker. The Feed
Rolls attached to the Feed Clutch drive a sheet of paper out of the paper tray and
down the paper path. As the sheet of paper travels down the paper path it strikes the
Registration Sensor. The Sensor sends a signal to the MCU PWB telling it that the
paper has arrived. The MCU takes the elapsed time from when it actuated the Feed
Clutch to the time it received the signal from the Registration Sensor and compares
that time with the set time (or expected time) that is stored in ROM on the MCU
PWB. If the time elapsed is within range, the print cycle continues. If the time
elapsed is slower than the set time or if there was no signal from the Registration
Sensor within the set time, the MCU interprets that as an error and sends a 200
Registration Jam status to the ESS PWB.
Processing image data
Although the MCU PWB does control and monitor the operation of the Printhead, the
actual video data sent by a host computer bypasses the MCU PWB on its way to the
Laser Diode PWB.
Example follows:
Video data originating from either a local computer or a network computer enters
the printer through the ports in the ESS PWB. The ESS PWB stores and buffers the
raw data and then converts it to rasterized data that controls the Laser Diode. The
rasterized data leaves the ESS PWB and enters the MCU PWB. The MCU PWB
does not buffer nor process the data, it simply passes it along as a video signal to
the Laser Diode PWB. The video signal switches the Laser Diode on and off
according to the image information sent in the raw data.
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