Printhead Evaluation and Assembly

Printhead Resistance Evaluation.

The Printhead Resistance Evaluation circuit is active only when the instrument is in motor speed calibration mode. When the motor speed calibration test runs, the Display/Recorder microprocessor starts energizing the printhead dots.

The measured printhead resistance used together with printhead temperature and voltage during printing determines the optimum duty cycle of the printhead. The printhead duty cycle is the time period that the printhead dots are energized; this is regulated by the width of the printhead strobe pulse, SB (see Figure 1—10, page 1—31). If the strobe pulse is too short, the print will be light; if it is too long, the excessive heat on the dots will greatly shorten the life of the printhead. The System microprocessor monitors the printhead resistance, battery voltage, and printhead temperature, and calculates the optimum printhead strobe width.

Monitoring the current through the printhead while increasing the number of dots turned on determines the printhead resistance. As the number of dots turned on increases, the combined resistance of the printhead decreases. Comparator U2A monitors the load on the VHD line at pin 5, through current sense resistor R27 and voltage divider R13 and R16. The VBATT line is monitored at pin 4 through voltage dividers RIO and R15. As the load on the printhead increases, the voltage across R16 decreases. When the voltage across R16 drops lower than the voltage across R15, U2A switches low. This directs the Display/Recorder microprocessor to stop sending strobe signals to the Recorder Gate Array at the STROBE input. The Display/Recorder microprocessor then determines the printhead resistance from the number of dots turned on and passes this information to the System microprocessor.

Display PCB/CRT Assembly

The Display PCB/CRT Assembly (Figure 1—11) is composed of a CRT andthe Monitor PCI3 Assembly. The Monitor PCB assembly processes video information and synchronization signals from the Main PCB and displays the information on the CRT. The incoming signals are Video 1, Video 2, and synchronization signals Frame Sync and Raster Sync.

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