Dräger 8000 IC Function Description page 24

2.2.2.19 Watchdog circuits (21)
Two watchdog circuits are used to supervise the program flow in the microprocessor. The
watchdogs are basically mono stable flip-flops that require to be re-triggered in a certain time
to allow normal operation of the Incubator. One of the watchdogs is triggered by the interrupt
driven routines, the other is triggered by the background software loop. Each of the watchdogs
is addressed by one unique address and is dimensioned to elapse in 100 ms. The software is
designed in a way that both watchdogs are re-triggered in time. Any hardware or software
failure would affect the proper re-triggering and therefore cause one or both watchdogs to be
activated. The outputs of the two mono stable flip-flops are combined and connected to all
relevant power drivers in order to deactivate any actuator thus transferring the incubator into
the safe mode. The signal transducer (22) is also activated independent of the software. The
watchdog mono stable flip-flops are tested in the 10-Minute-Test.
2.2.2.20 +5 V voltage monitoring (19)
The digital supply voltage of 5 V is monitored by a window comparator for high and low voltage
conditions. The limits are set to 4.4 V and 5.5. V. If the window comparator senses high or low
voltage, the 24 V power supply for the solid state relays and the oxygen valves is interrupted
by the relay (17). Via transistors V15 and V16 the comparator limits to test the function of the
voltage monitoring can be adjusted. This is done by the software via ports CA2 and CB2 of PIA
S1 (10-Minutes-Test).
Normal operation:
Test upper limit:
Test lower limit:
The +5 V supply voltage is also monitored via a watchdog circuit (25A) on the CPU PCB (25).
This watchdog circuit cannot be tested during operation because it triggers a reset.
2.2.2.21 Driving the actuators
Both air heater and humidifier are digitally controlled through the PIA1 and the solid state
relays (13) and (14). The driver signal for the humidifier is read back by PIA2. The valve (Inc.
8000 IC only) is controlled by PIA1 and driven by a power transistor on the Motherboard PCB.
The driver signal is supervised by the second PIA.
2.2.2.22 Monitoring of air heating
The driver and status of the air heating are monitored by an insulated current sensor at the
secondary side of high voltage. The output signal is monitored via PIA2.
6141.22X Incubator 8000 IC/SC/NC
CA2 = L-level; CB2 = H-level
CA2 = H-level; CB2 = H-level
CA2 = L-level; CB2 = L-level
Function Description
09.99 Page 24 of 49