Mallinckrodt NELLCOR NPB-4000 Service Manual page 144

Section 14: Main Color Board Digital Theory of Operation
14.2.2 Microcontroller Signals
14.2.3 MEMBRANE SWITCHES/POWER SUPPLY
14.2.4 POWER SUPPLY GENERATED LED SIGNALS
14.2.5 BATTERY SIGNALS TO THE A/D CONVERTER
14.2.6 SYNC/ALARM 50KHZ SIGNAL
14-4
The power supply circuit and microcontroller have 2 signals which connect
directly with each other, EARLY WARNING and PSOFF. The battery circuits
generate an EARLY WARNING signal to the microcontroller to warn it that the
power is going down within 100 ms. This EARLY WARNING signal is
generated if the power On/Standby switch is pressed or the battery voltage gets
too low. The signal is connected directly to INTERRUPTS 5 and 7. The
software will save any information it must save and finish its housekeeping
before sending the PSOFF signal back to the battery circuits.
The power supply circuit connects directly with the On/Standby switch and the 2
LED' s on the membrane switch. The On/Standby switch goes directly from the
membrane switch to the power supply board via the 50 pin cable. This is to
allow powering up when off. Another switch also goes to the battery circuit, the
Alarm Silence switch. Unlike the On/Standby switch, this switch also goes to
the microcontroller.
The power supply circuits generate two LED driving signals, one that indicates
that the AC mains is connected, and one which indicates that the DC power is
connected. When the AC MAINS LED is being driven on, a high voltage level
goes to the 386EX port 1 bit 6 as the AC mains input status bit. When the LED
is off the signal is a logic low. The software will interpret the signal levels so as
to tell whether the AC mains is connected or not. The same is true with the DC
power input connector. The signal driving the DC led goes to port 1, bit 7. This
indicates the DC power status.
The battery voltage signal is sent to the main board where it is buffered by an
amplifier circuit which converts it to a signal in the 0-5 volt range of the A/D
converter. The BATTSNS+ signal is sent directly to the main board and is
divided by 2 by a resistor divider circuit on the positive input to the amplifier.
The amplifier has a gain of 2, which brings the signal back to its original signal
level and then we resistor divide it to the 0-5 volt range for the A/D converter.
The power supply needs a 100 kHz signal to synchronize its own internal
operation and to keep the alarm from sounding. The 100 kHz signal is "anded"
with the watch dog timer (WDT) and if the WDT should time out the power
supply circuit will sound the alarm and shut down the main board.