Novametrix Medical Systems OXYPLETH 520A, Руководство по обслуживанию

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2471 & 2775 Main Board
5-Jun-01 Model 520A Service Manual
21
PRELIMINARY
The +2.5 volt output from IC35 pin 6 is fed to the non-inverting input of amplifier
IC36 pin 3. Resistors R32 and R33 combine for a gain of 1.617 that provides a
+4.096 volt reference (approximately) supply, 4VREF, at IC31 pin 1.
The 4VREF is fed to IC30 pin 13 which is set up as a unity gain inverting buffer
amplifier, therefore the output at IC30 pin 14 is -4 volts. This -4 volts is used by
IC29 as a reference voltage for VLED (Voltage LED) and CNTRST (Contrast)
controls. (See sheet 4 on 2471 schematic, sheet 3 on 2775 schematic.)
Preserving RAM and Real Time Clock Data
9.3.4
The NAND gate output at IC11 pin11 will be Low when the monitor is on (IC10
pin 2 is High) and High when the monitor is off (IC10 pin 2 is Low). (See sheet 4
of 5 on schematic.) This PWRON* (Power On) signal is used to prevent corruption
of RAM and clock data when the monitor is turned off. It does this by going High
and therefore denying CPU access to the RAM and clock chips so that as the power
supplies crash when the monitor is turned off, the CPU cannot send erroneous data
to the chips.
Whenever the CPU is writing information to the RAM or Real Time Clock chips,
the CPU momentarily sends the OFFDIS (Off Disable) line High. The High going
level appears at IC11 pin 2. Since the monitor is powered on, IC11 pin1 will be
High. This means IC11 pin3 momentarily goes Low, Q6 starts to conduct and IC10
pin 4 goes High. In this reset condition the /Q1 output #1 flip-flop of IC10 (pin 2)
will be held high even if the user presses the
POWER
key and clocks the flip-flop.
In effect, the CPU is not allowing the monitor to be turned off. The Low at IC11
pin3 will last for the duration of the RC time constant set by C21 and RP4 (pins 5,6).
These values were chosen to produce a time-out longer than the time necessary to
complete the write to RAM or Real Time Clock operation. After the RC time-out,
IC10 pin 4 returns Low and a press of the
POWER
key will toggle the /Q1 output of
IC10 and the monitor will turn off. This is done to prevent corruption of RAM and
Real Time Clock data.
Low Battery Voltage Shutdown
9.3.5
The CPU monitors the battery voltage and provides the user with a low battery
indicator (
), messages and alarms. However, if these are ignored, a hardware
circuit will take over and shut off the monitor before the battery is damaged.
The pulse width modulator IC9 requires at least 7.6 volts at pin 7, its voltage supply,
in order to operate. This pin typically draws 10 mA of current. The resistance of the
R63 and Q8 combination is approximately 114 ohms. This equates to a voltage drop
of approximately1.14 volts. Therefore if the battery voltage drops under 9.0 volts
(approximately), IC9 will not have sufficient voltage to operate and will shut down.
Shutdown of IC9 stops current flow through transformer T1 and the secondary
supplies shut down, effectively turning off the monitor.