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Technical Supplement
The large bulk capacitor C3 is required due to the large current spikes that are
required by the printer and the large internal series resistance of disposable
batteries. Bulk capacitance is required to lessen the battery voltage drop caused
by the current spikes.
The N-20PA has three additional user-control buttons. L8, L9, L10, C120,
C121, and C122 provide ESD protection. R103, R104, and R105 provide
pull-ups when the user buttons are open. These pull-up resistors are in the
printer interface circuit to ensure that the buttons (ON, ADV, and D/D) are never
left floating, regardless of whether an optional printer flex circuit is installed or
not.
The optional user controls consist of three momentary push-button elastomeric
contact switches. Pull-up resistors are provided by the printer interface circuitry.
R1, R2, and R3 help protect the input port by providing some current-limiting
capability. C4, C5, and C6 debounce the switch contacts.
9.7
CIRCUIT ILLUSTRATIONS
The foldout graphics referenced throughout the Technical Supplement text are as
follows:
Figure 9-8: LED Drive Circuit
Figure 9-10: N-20PA HSO Timing Diagram
Figure 9-13: AC Variable Gain Control Circuit
Figure 9-15: CPU Circuit
Figure 9-20: Output Port Circuit
Figure 9-23: Display Control Circuit
Figure 9-25: Power Supply Circuit
Figure 9-26: Power Control Circuit
Figure 9-32: Printer Interface Circuit
Figure 9-33: Printer Flex Circuit
Figure 9-34: N-20PA Main PCB Schematic Diagram
Figure 9-35: N-20PA Auxiliary PCB Schematic Diagram
Figure 9-36: N-20PA Flex Circuit Schematic Diagram
9-31
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