18
19
Each HP version probe, therefore, has its specific resistor to indicate to the
microprocessor the correction to be taken.
Figure 8-1. Wiring Diagram for 392HP
Figure 8-2. 4-Pin Probe Connector Expanded View
8.4 Rate of Change with Hot and Cold Peak Hold (Order Option R)
This unit has microprocessor program revision E which can display any one
of four different measurements repeatedly. The four display modes are as fol-
lows:
1. Normal Temperature. The temperature as sensed by the attached
probe is displayed with updates 2.5 times per second. (Switch Position “N”).
2. Rate of Change of Temperature. The change in temperature
sensed is updated every six seconds. The rate of change is displayed in
degrees per minute with proper polarity sign.
3. Hot Peak Hold. Continuous display of the hottest temperature
sensed since the last time the unit was turned on. (Switch Position “HP”).
4. Cold Peak Hold. Continuous display of the coldest temperature
since the last time the unit was turned on. (Switch Position “CP”).
If either the cold peak or the hot peak should increase, the display
will be updated to read the new colder or hotter peak. Turning the instrument
off will remove the cold and hot peaks from memory. Upon turn on, the
microprocessor is reset for the input of new data.
9.0 THEORY OF OPERATION
9.1 General
The microcomputer, as directed by this program receives and evaluates data
from the analog-to-digital converter, and delivers the proper temperature val-
ues to the display driver. The readings may be in Fahrenheit or Celsius, peak
or current value.
The Flow Chart (Figure 9.1) shows the operation of the program. Details of
the components will be discussed in this section.
9.2 Program Flow
When Power is turned on, the program will start executing instructions from
address 000, the location of the top level program labeled “Main.”
The first routine called is “Initialize”. This is a house-keeping routine which
establishes the starting conditions. The data memory (RAM) is cleared, the
input/output ports are configured, and the display is set up for 0.0 reading.
Next, the routine “Gaincheck” is called. A solid state switch is confirmed to
connect the AID converter to a precision wire-wound resistor of known value.
The resistor reading is compared with the ideal value, stored in memory, and
a factor is calculated and stored to be applied to subsequent readings of the
probe resistance.
Probe and gaincheck readings are maintained in a “Moving Average” (M.A.)
fashion to minimize the effects of random variations. Gaincheck readings fol-
lowing “Initialize” or the detection of a removed probe cause the moving
average to be crammed with the current value. Subsequent passes cause the
moving average to be updated and refined. Gaincheck updates occur alter-
nately with probe readings the first 15 cycles, and then they are performed
only every 20th cycle. This is to allow more computer visibility to the probe
updates with reduced inertia. Settling time is provided to allow analog refer-
ence circuits to stabilize when the probe readings are done on the high scale.
The compensation readings are always made on the low scale.
If readings exceed the compensation limits (about 6% is allowed), the pro-
gram branches to a subroutine which displays “Help” on the second such
excess.
The next routine entered, “Get Probe”, performs as described above for the
compensation readings; however a separate moving average is maintained
