The position of the print head is sensed by the signal PR HOME. Whenever the print head is not in the home
position, a switch in the printer closes, shorting PR HOME to switched ground. Whenever the print head is in
the home position, the switch opens, allowing R118 to pull PR HOME high.
The print head dot pattern and pulse width are controlled by the CPU. The proper printer dot values are loaded
into the output port, then the proper pulse width is loaded into the CPU CAM for PR STROBE. The signal PR
STROBE enables the outputs for the specified pulse width. When the PR DOTX lines are high, a dot will be
printed.
Printer Flex Circuit and User Controls
The thermal printer is plugged in via connectors JP10 and JP11. The PR PRESENT signal is connected to
switched ground to tell the CPU that a printer is installed. U1 is a Darlington pair driver chip that is used to drive
the printer dots and motor. When an input is high, the output is shorted to the ground, driving the output load.
Constant power (VPRN) and a power control line (PWR ON) are provided by the auxiliary PCB. Q1 is used as
a power control FET. Both halves are used in parallel to reduce the on resistance. When PWR ON is high, the
sources (S1, S2) short to the drains (D1, D2), connecting ground to U1 and C3. PWR ON also controls the
regulated power supplies; thus, Q1 and the power supplies are both enabled and disabled at the same time.
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 by the current spikes.
The N-20P 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 elastometric 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.
12.3 Calibration Verification for N-20P
CAUTION
: Adhere to all testing instructions; failure to do so may damage the N-20P.
Overview
The N-20P is powered by alkaline batteries. The N-20P design includes built-in electrical insulation, no ground
resistance or electromagnetic leakage testing is required. In addition, the N-20P required no calibration.
Required Tools and Equipment
The following tools and equipment are needed to perform test procedures.
•
SRC-1 pulse oximetry tester
•
Variable DC power supply; 1 amp supply, voltage range 0-6 VDC
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