20
counter IC36 to provide the 1.23 MHz ADC clock. Communication between microprocessor IC22 and all bus
devices is provided by a 24-bit address bus, 8-bits data bus and the three wire serial peripheral interface
(SPI) bus. The system processor is equipped with a watchdog timer and reset circuit.
The system program is stored in a 4 Mbit FLASH memory IC28 as 512k by 8 bit words. Program memory is
provided by a low power 1 Mbit SRAM IC32 formatted as 128k by 8 bit words. Non-volatile settings are
stored in an EEPROM IC26 which talks to a dedicated serial port on the system processor. All bus devices
are powered from the +5VD digital supply, except for the RAM that is also powered from the VRAM supply
when the system is in “standby” mode.
Should the software detect a system fault or fail to reset its watchdog timer, the system processor will output
a logic low level on its watchdog output WDOG. This is latched as FAILSAFE by the output of the bistable
formed by IC31, allowing the output of the relaxation oscillator IC1 to pass through IC4 to sound the piezo
sounder LS1. The FAILSAFE signal is also used to disable the isolated power supply module PM100.
The power supply board battery backup voltage BATT_BACKUP, derived from either NiCd battery or an
external supply, appears on the supervisory circuit IC35. This supervisory circuit has two functions. First, it
generates Reset and Halt signals for the system processor when the 5 volt power supply powers on and off
respectively. Secondly, it controls transistor TR8, allowing the 5 volt line to power the RAM chip IC32 and
PIC when the system is “on”, or the BATT_BACKUP to power the RAM and the PIC when the system is
“off”. This allows values to be saved in the otherwise volatile RAM and to maintain the low power standby
mode of the PIC.
Additional control signals from the system processor are decoded by IC16, which generates seven chip
select lines and IC17 which generates latch outputs for the LED display drivers. The data bus D0 ~ D7 is
filtered by inductors L12~19 to produce a buffered data bus DB0 ~ DB7 for the display drivers.
There is also a PIC micro-controller IC5 on the power supply board 8620AB, which has its own independent
watchdog, reset circuit and provides the real-time clock function. The PIC provides control of PSU status,
battery charging, printer and alarm/annunciator audio output. Communications between the system
processor and the PIC are continually monitored in order to provide additional fail-safe security for the
monitor. All measurement, alarm and communications ports of the instrument are isolated to IEC601.
2.4.2 Temperature System
The temperature system for the Model DINAMAP
™
Compact
Monitor for reading predictive thermistor probes
is located on the Main Board.
If a thermistor probe is used, the resistance of the probe varies in relation to temperature. The probe
resistance is effectively in parallel with R63 and therefore a change in temperature results in a change in
voltage applied to input selector IC19. The selected input voltage from IC19 is connected to the voltage to
frequency converter (VFC) IC18. The frequency of the square wave output from the VFC varies
proportionately to the input voltage. The output signal is isolated through opto-coupler PC2 and enters the
system processor, as PRED_TEMP_F. The processor receives the square wave signal and determines the
temperature by counting the number of edges received per unit interval.
Through the application of a software algorithm, the system processor provides two select lines TEMP_SEL0
and TEMP_SEL1. These are isolated by opto-isolators PC3 and PC4 and used to control the input selector
IC19, allowing the appropriate resistor tree to be selected. There are four selectable channels, two providing
calibrations points, a third used for probe type detection and the fourth for the thermistor measurement.
The probe type is determined by measurement of the frequency produced by the VFC from a DC input
derived from resistor tree R59-R61; for an oral probe this produces a frequency equivalent to 42 degrees
Celsius, Rectal probes short out R61 and hence change the frequency slightly. The software is capable of
determining which probe type is attached and hence selects the appropriate measurement mode.
