Datex-Ohmeda Anesthesia Delivery Unit S/5 Technical Reference Manual page 342

Datex-Ohmeda ADU
All valve driver circuits have current limit circuits on the supply side. The product of the short-circuit
current and supply voltage is limited below 10 VA, so there is no risk for sparks and therefore fire, in
case of an output short to ground.
A reset causes the outputs to go into off state. On proportional valve drivers this is performed by
disabling the positive supply side on the current limit circuit. This means that the inspiratory valve will
be closed while the expiratory and peep valves are open. The pressure from the patient circuit is thus
released.
Auto/Man. switch input
The input for the Auto/Man. switch has two digital lines with pull-up resistors. External switches
connect the lines to ground.
Analogue input
For analogue inputs there is a 16-bit A/D-converter on the board. Two eight channel analogue
multiplexers are required to select and measure all input voltage channels. In addition to the
pressure and flow signals, the supply voltages are also measured for self diagnostics.
The input voltage of the analogue channels of the board, range from 0 to 10V. It is divided by two in a
resistor network to convert it into the input voltage range of the A/D-converter. The A/D-converter
uses +5V reference voltage which also is employed in generating the +10V supply for the pressure
sensors.
Back-up processor
The back-up processor ensures patient safety by releasing high patient circuit pressures in the Auto
mode. The back-up processor opens the expiration and PEEP valves and closes the inspiration valve
if the pressure in the patient circuit rises above the limit set by the User, +20% or 5 cmH
approximately.
The back-up processor is located in the serial data line between the central electronics CPU and the
ventilator CPU. The back-up processor picks out the information from the messages that the central
electronics CPU sends to the ventilator CPU. The needed information is the state of the ventilator CPU
and the pressure release limit value. The back-up processor receives the pressure information from
the two pressure sensors in the patient circuit.
Figure 15
C-16
The back-up processor
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Document No. 8502855