2/Theory of Operation
2-14 1503-0151-000 3/27/97
External Interface
The external interface of the ventilator is an important design task from the stand-
point of Electromagnetic compatibility (EMC). It is important to protect the ventila-
tor from conducted and radiated Electromagnetic Interference (EMI) and from
Electrostatic Discharge (ESD). In addition, EMI design precautions are taken to
control the emission of EMI via cabling and access ports.
Communication Interface (RS232C)
There are two RS232C interfaces. Both operate with a maximum baud rate of
38.4K baud. The isolated interfaces eliminate the possibility of ground loops.
External Interface Connector
This section of the design covers the various inputs and outputs that pass though
this connector. Each input or output is protected from EMI and ESD in an appro-
priate manner.
Sealed Lead Acid Battery
A sealed lead acid battery provides battery backup operation for the 7900 ventila-
tor. Batteries meet the following requirements:
1. Capacity to operate unit for 30 minutes.
2. Good reliability history with Ohmeda.
3. Long float charge life.
4. Battery pack is internally fused - in line replaceable.
The 7900 is not a portable unit. Batteries for the 7900 Ventilator are used as back
up power in case of a power failure. Therefore the battery is in a float charge state
most of the time.
Battery terminals and connecting wires are protected against short circuits.
Input
Nominally 13.7 Vdc at 25
o
C during float charge.
Output
+10 to +14.8 V dc during discharge
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