SLE 2000 HFO, Service Manual

Page: 63 / 258

Issue 10 (01/10/2004) Page 63 of 258
15.0.4.4 Dual Fan Fail Alarm
This circuit operates audible and visual alarms if the speed of either fan motor falls below
approx. 75% of normal rate. Current pulses from the motor commutation develop pulses which
are compared with a reference voltage in U23A and B. This results in output pulses from U23/
1 and U23/7 for each fan. The negative going edges trigger U22 which remains re-triggered
(logic 1) as long as the interval between triggered pulses is greater than 5ms. The data output
of U22 is fed to U24 and is latched in the latter by the positive going edge of the pulse from
U22. The output of U24 remains at logic 0 as long as the interval between pulses from the
motor is less than 5ms. However, if the motor runs slowly or stops or is open circuit then the
output of U24 goes to logic 1 and enables the fan fail alarm via D2 or D3. MOS1 and MOS4
hold the reset pins of U24 inactive as long as there are motor pulses. U24 is reset during
power up or if motor pulses fail and enables the fan fail alarm.
15.0.4.5 Oxygen Alarm
Introduced with version 2.1 of display firmware and addition of connecting wire to link PLA29b
to PLB29b. See “CD/W0308 Wireloom Issue 6” on page 152. (See “SI 990305
Addition of the
oxygen alarm on SLE2000 HFO” on page 218.)
15.0.4.6 LED Bargraph
This circuit simultaneously displays, on a 40 segment bargraph module, low, fail to cycle, and
high alarm pressure thresholds as well as the instantaneous pressure signal (derived from the
A0702/04 buffered transducer output). These four input voltages are first buffered and
conditioned before being time multiplexed via the 4066 analogue switch (U31). Oscillator U19
drives a counter (U18) which is used to sequentially enable U31A,B,C and D. The multiplexed
signal is then used to drive the bargraph PCB (A0737).
15.0.4.7 Processor System
All alarm parameters are determined by potentiometer voltages representing fail to cycle, low
and high pressure thresholds. Also Timebase and pressure range switch settings, and CPAP
mode status, patient circuit pressure, HFO motor speed are all represented by voltages which
enter the processor system via addressable ADC0809 (U5)
U7 and its peripheral passive components form a one second interrupt generator which is
used by the processor system to determine a 1 minute period for the screen freeze function.
The watchdog device U12 will provide a reset pulse for the processor (U10) on power up,
power down and brownout conditions. It will also monitor U13/13, (VIA- Versatile Interrupt
Adapter) and if no change of state is recognised on this line (indicating a processor fail) within
1.6 seconds then the watchdog will provide a reset pulse for the processor. Whilst no pulse is
recognised on U13/13 the watchdog will also provide a signal out on U12/14 indicating HFO
CPU (A0736/030736/02) system fail which is sent to the Alarm PCB.
15.0.5 Bargraph Display Board Assembly (A0737)
This board accepts a time multiplexed signal from the HFO main Board (A0736/030736/
02)which contains the signals for the high, fail to cycle and low alarm thresholds along with the