Welch Allyn SureTemp 767 - Probe Identification Logic; Basic Function; Circuit Operation; Figure 4 - Probe Logic

Technical Manual
17
During normal operation, software turns the warmer circuit on and off. The width of the pulse on
HTRC determines how long Q5 is turned on, thereby determining how long the heater is heating.
Once the HTRC pulse goes high again, the base of Q5 is pulled high turning it off, and the
capacitor discharges to VCC through D1.
Q13 and Q14 are selected for their low saturation on voltage. D1 is a diode clamp used to keep the
base of Q5 from attaining a much higher voltage than VCC. R4(47K) and R2(4.7K) in combination
with R3(47K), serve as pull down resistors ensuring that the processor feedback lines (U1-31 and U1-
32) go low immediately upon warmer component shut off. C2 serves as an RFI suppression
component. C35 is not populated.
Probe Identification Logic
Basic Function
The probe logic circuitry is needed to determine which type of probe is connected to the device. This
allows the microprocessor to initiate the proper algorithm for converting a thermistor measurement
into a temperature. Figure 4 shows the probe logic diagram and corresponding circuitry.
PROBE LOGIC
SIGNAL ORAL RECTAL CAL KEY NO PROBE
PROBE_0 0 0 1 1
PROBE_1 0 1 0 1
PROBE 1
PROBE 0
VCC
1
2
R10
1M
1
2
R11
1M
1
2
C19
0.01UF
1
2
C20
0.01UF
J1-6
J1-3
C U T H E R E
F O R R E C T A L
C U T H E R E
F O R C A L K E Y
Probe Connecto
r
PCB
To Micro
p
rocessor
N
O CUT FOR ORAL
FIGURE 4 - PROBE LOGIC
Circuit Operation
R10, R11 (both 1 MW pullup resistors), J1-6(PROBE_1), J1-3(PROBE_0) and J1-5(GND) are used to
provide logic 0 or 1 inputs to U1-17 and U1-18 depending on whether a probe or cal key has been
plugged into the Model 767 Enhanced probe connector receptacle. When a probe has been plugged
in, the software determines whether it is Oral, Axillary, Rectal, or a Cal Key as follows:
· When J1-3 (CAL) and J1-6 (RCTL) are both connected to J1-5 (GND) the software determines
that the probe is an Oral probe;