Sigma 6000+ - Page 25

THEORY OF OPERATION (Continued)
Axe Circuit (Continued)
When Q10 is pulsed on, any charge that has accumulated on capacitor C38 due to current supplied
through R38 is shorted to ground. Thus, U22 pin 1 is continuously held LO while the pulses are present
at pin 17 of the CPU. The end result is: pin 3 of the CPU, the CLR pin, stays HI.
However, if the program or the microprocessor malfunctions, causing a cessation of pulse output from pin
17 of the CPU, the following events will occur:
1. The voltage on C38 rises at a rate determined by the values of R38 and C38.
2. When voltage at U22 pin 1 rises past the point where the inverter functions by driving U22 pin 2
—. LO, the circuit consisting of U22 pins 1 and 2, C46 and R49 begins to oscillate.
3. This changing voltage, if applied from U22 pin 2 to U20 pin 2, will function as an inverter. The
inverted output is fed to.a second XOR gate which also acts as an inverter; its output goes to pin 3
of the CPU. Logic LO stops and resets the microprocessor.
4. The output signal also goes to the base of Q9 through R47 and C41 switching it off. This triggers
the AXE circuit.
5. When Q9 turns off (Q9 is on during normal pump operation), the voltage at the gates of Q2 and
Q5 change from LO to HI, thus turning them on. `
When QS turns on, it pulls the base of Q6 LO, thus turning it off.
When Q2 turns on, it pulls U8 pin 3 LO, turning the latch off. This sets pin 1 LO which turns off
Q4. Q2 also sends a signal to the microprocessor through D9 telling it that U8 is being turned
off.
8. With O4 and O6 both off, Q7 turns off and power to the unit shuts down.
Na
- Display Board
The display board consists of two 4-digit intelligent display modules, a keyboard decoder, an
amplifier/demodulator for the ultrasonic receiver, a LED to indicate the unit is plugged into AC and an
A/D converter for the demodulated ultrasonic signal.
The display modules are intelligent LED’s that convert information from the microprocessor to a visual
form. The micreprocessor loads through software procedures, via the data bus, each digit with
information in turn. The coding on the bus is 7 bit ASCII. There is also a blinking control to the modules
tied to pin 18 of both displays. This is an active low signal supplied from U19 pin 64 on the main PCB.
IC4 on the display board is a 74C923 keyboard decoder that converts the membrane key pushed to a 5 bit
. hexadecimal signal and sends it to the microprocessor. When a key is pushed, a data available signal is
sent to the microprocessor flag line from IC4 pin 13. When the microprocessor is ready to read the data,
it sends an output (active low) to IC4 pin 12. Capacitors C1 and C2 control the keyboard scan frequency
„and the keyboard debounce time.
IC2 and IC3 are analog amplifiers that increase the level of the received ultrasonic signal by 100 (10x10).
The output of IC2 is fed to the demodulating, voltage doubler, and then to IC1, an A/D converter. TH1,
REF1 and R18 from a circuit which temperature compensates the ultrasonic sensor system.
IC1 is an 8-bit converter, controlled by the microprocessor. A signal from the microprocessor starts the
conversion process at the right time, and the converter signals the microprocessor via the interrupt
function when conversion is complete. The microprocessor then signals the converter to lad the voltage
information to be read onto the bus.
Section 1 Page 17