Ohmeda 3000 - Page 9

1/Functional Description
The ADC 3711, U6, uses a pulse modulation analog
to digital conversion technique. The conversion rate
is set by the frequency of an internal oscillation
whose frequency is determined by the external com-
ponents R4 and C14. The exact oscillator frequency
is not critical and may vary by + 15% from the
nominal 400 kHz. The oscillator frequency may be
measured on pin 18 of U6. With a nominal 400 kHz
clock frequency, conversions within the ADC 3711
will take place at an approximate rate of 3 per sec-
ond.
The ADC 3711 will output BCD data on demand in
accordance with the coded digital signals applied to
the digit select inputs DO and D1, pins 20 and 21 re-
spectively. The data latch enable is tied “Low”,
therefore, the BCD data of the A/D converter will be
output to the microcontroller through 8243 #2 in
conformance to the following codes that are applied
to the digit select inputs:
Do D1 Selected Digit
L L Digit 0 LSD
L H Digit 1
H L Digit 2
H H Digit 3 MSD
Note: The magnitude of the selected digit is present
at pins 23 and 24,
The ADC 3711 is continuously converting the analog
voltage present at its input to a number of counts
between 0 and 3989 (BCD format). Therefore, the
start conversion, input at pin 7, and the conversion
complete, output at pin 6, are misnomered. The
start conversion input only controls the transfer of
information from the internal counter to the digital
latches. The conversion complete output goes to a
logic “Low” on the rising edge of the start conver-
sion pulse which is issued by the microcontroller.
The conversion complete will go to a logic “High”
sometime later when the new conversion informa-
tion has been transferred to the display latches. The
start conversion pulse may occur at any time in the
conversion cycle because the microcontroller is run-
ning asynchronously to the A/D clock. Therefore,
the amount of time from the start to finish will vary.
The maximum time difference between the start
conversion and conversion complete pulses in this
application is about 300 msec.
The operation of the temperature and line voltage
Measurement circuits can be summarized as fol-
lows: The analog voltage signal derived from a vol-
tage divider network and a precision reference
source is directed to the input of the A/D converter
through an eight channel analog multiplexer. For
the line voltage measurement, the voltage source is
obtained from the rectified, filtered, and unregulated
output of the power transformer. Switch selection is
software controlled by the microcontroller which
toggles the A, B, and C input lines of the multi-
plexer.
The analog voltage is converted in the ADC 3711 to
a digital signal in four digit BCD format (0 to 3999
counts). The microcontroller sends a start conver-
sion pulse to the ADC 3711 which then starts to up-
date the digital data in the output latches. When all
of the counts have been internally transferred, the
A/D converter toggles the conversion complete out-
put line. The microcontroller then reads the indi-
vidual BCD digits using coded signals to the digit
select lines of the A/D converter.
ADC Calibration
The A/D converter is calibrated by connecting a
5900 + 0.1% ohm resistor to the patient probe jack
and placing the DIP switch on the control board in
the following position:
Swritch 41 Open (Off)
Switch 42 Open (Off)
Switch #3 Open (Off)
Switch #4 Closed (On}
Potentiometer R44 on the control board is then ad-
justed until the elapsed time display reads exactly
1122. With the DIP switches in the given position,
the patient temperature display will read out the ac-
tual patient temperature, even if it is outside of the
normal range and the control temperature display
will read out the percent of nominal line voltage.
During operation, the calibration of the A/D conver-
sion system may be checked by pressing and hold-
ing the hidden switch located above the alarm si-
lence switch on the control panel. After 2 seconds,
the patient temperature display should read 25.0
and the control temperature display should be 37.9.
The elapsed timer should read the nominal line vol-
tage + 2%.
Microcontroller
The control system is located in the 8031 microcon-
troller. It operates at a clock speed of 6MHz and can
he verified by measuring the frequency at the Ad-
dress Latch Enable (ALE) pin to be 1 MHz ("On" =
0.33 usec and “Off” = 0.67 usec). Grounding the EA
pin enables the 8031 to execute instructions from an
external memory device.
When the microcontroller performs a read instruc-
tion from EPROM, the low order address (8 bits) is
output from Port 0 while the high order address (6
bits) outputs from Port 2. (Note: Bit 6 is configured
only to provide expansion compatibility with a
27128 EPROM). The ALE pin goes “High” allowing
the LS373 to appear transparent between the
EPROM and the microcontroller. After the ALE out-
put goes “Low”, the low order address is latched to
the outputs of the D flip flops within the LS373.
This allows the 2764 to remain addressed by the
microcontroller, and return 8 bits of data while
using only two ports.
Port 1 of the 8031 is used to communicate to the
three 8243 I/O expanders. Bits 5-7 are connected to
the Chip Select (CS) line of the first, second, and
third respective 1/O expanders. Providing a “Low”
signal on one and only one of the outputs activates
the corresponding IC. Bits 0-3 hold the instruction
to be carried out by an 8243 when the enable bit 4
transitions between “High” and “Low".