Datex-Ohmeda Aestiva 7900 SmartVent, Service Manual

Page: 35 / 181

2/Theory of Operation
1503-0151-000 3/27/97 2-17
Program Memory
Flash EPROM
Two 128K x 8 Flash memory devices are used, with on-board expansion capability
to 512K x 8 devices. This memory contains the real time operating system (RTOS)
and software code. The Flash memory devices are socketed.
System RAM
This memory consists of two 32K x 8 CMOS static RAMs with on-board expansion
capability to 512K x 8 SRAMs.
Non-Volatile Memory
This memory consists of a single 2K x 8 EEPROM and stores information which
needs to be retained when the system is powered down. This includes user se-
lectable operating parameters and a system error log.
Safety Relevant Computing
This section addresses the Safety Relevant Computing (SRC) requirements of DIN
V VDE 801. The electronic hardware design provides the necessary capability for
meeting these requirements. This includes:
• Digital monitoring of selected control signals
• Analog monitoring of supply voltages, internal control voltages,
feedback signals from the flow and inlet valves, and battery voltage
• Automatic switch-over to battery operation in the event of an inter-
ruption in ac power
• Use of the software watchdog timer in the 68340 processor for tem-
poral monitoring, with direct de-energizing of the flow and inlet
valves in the event of a non-recoverable error
• Use of an operating mode watchdog for logical and temporal moni-
toring, with direct de-energizing of the flow and inlet valves in the
event of a non-recoverable error
• Monitoring of the system clock to detect an operating frequency out
of an acceptable range
• Use of a redundant high pressure limit safety switch in the bellows
drive circuit which directly de-energizes the flow valve