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9.6.13 Printer Control
9.6.13.1 Printer Interface Circuit
The nominal voltages and voltage discharge curves are significantly different
between rechargeable and disposable batteries. In order for the CPU to predict
how much "battery life" remains, the nominal voltage and discharge curves must
be known; the BAT_TYPE signal provides that information
Printer circuitry is divided into two subsections: the printer interface and the
printer flex circuit.
•
Printer interface circuit (auxiliary PCB) — This circuit detects the
presence of the flex circuit, and supplies power to the print heads and paper
advance motor. Noise generated by the printer motor is filtered. The
circuitry is protected from excessive battery currents by a fuse.
The printer interface circuit is illustrated in Figure 9-32, at the end of Section 9.
•
Printer flex circuit — The printer flex circuit is added when the printer is
present. The printer generates a timing signal that is read by the CPU and
sent to the flex circuit. This circuit signals the CPU that a printer is present
by connecting one CPU input to ground. Power and power control signals
from the auxiliary PCB generate an output load for a resistor array; heat
from this process produces a dot matrix pattern on thermal paper.
The printer flex circuit is illustrated in Figure 9-33, at the end of Section 9.
The printer control block diagram is shown in Figure 9-6.
User control is provided by momentary push buttons: ON (on/off), ADV
(advance), and D/D (day/date). ON enables or disables the printer, ADV controls
the advance of printer paper, and D/D sets date, time, and other clock
parameters.
When a low battery voltage condition is present, the N-20PA adjusts power to
the printer's head; however, a weak battery voltage condition causes the printer
to shut off, thereby allowing the N-20PA to continue to display oxygen
saturation and pulse rate readings until the batteries are exhausted. An ambient
temperature sensor adjusts printout quality to compensate for environmental
conditions.
The printer interface circuit is illustrated in Figure 9-32, at the end of section 9.
The following is a description of the printer interface circuitry found on all
N-20PA auxiliary PCBs.
The CPU reads the PR_PRESENT signal to determine if a printer is installed.
With PR_PRESENT left floating, it is pulled high by the weak pull-up resistor
inside the CPU. If a printer is installed, PR_PRESENT is connected to switched
ground, which causes a low input to the CPU. The optional printer circuit is
protected from excessive battery currents by fuse F2. CR28 is used to block
noise generated by the printer motor being injected onto the batteries.
The N-20PA printer is a 16-character-wide thermal dot matrix printer, which
generates a CPU interrupt for every dot column. The thermal energy given to the
print head is controlled by the pulse width of the active high signals PR_DOTx.
In order to provide consistent print quality, the ambient temperature, print drive
voltage, and print head resistance must be measured and accounted for.
Technical Supplement
9-29
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