HP 3457A Service Manual page 192

As the power supply reaches operating voltage, the voltage at the plus input of U636a exceeds the
e reference voltage at the minus input and the output goes positive. The positive output of U636a ap-
plied to the plus input of comparator U636c causes its output to go high. The high output of U636c is
coupled through inverters U634a and U634b to the RESET input of the microprocessor to release it from
the RESET state.
Low Power Reset. If the un-regulated +5 volt power supply should drop below approximately +7
volts, the plus input of comparator U636a drops below the +3 volt reference at the minus input causing
the output to go low. The low output of U636a is applied to the plus input of U636c causing its out-
put to go low. The low output of U636c is coupled through inverters U634a and U634b to the RESET
input of the microprocessor. The processor is held in a RESET condition until the un-regulated +5 volt
supply returns to normal.
Microprocessor Inoperative Reset. Under certain conditions it is possible for the microprocessor 10
"lock-up" or become non-operational. During normal operation, the microprocessor continually toggles
the OTL signal at U6I1, pin 19. If the OTL fails to toggle, the processor is reset by the Watch Dog circuit
as follows:
A continually high or low OTL signal applied to capacitor C631 allows the input of gate U633a to go
low. The high output of U633a and the normally high RESET signal applied to the inputs of gate U633b
cause its output to go low and reset counters U631, U632a and U632b. After being reset the counters
begin counting the 2 MHz clock signal applied to U631, pin 10. After counting for slightly more than
0.5 seconds the output of counter U632b pin 9 is set high. The high output of U632b is applied to the
minus input of comparator U636b causing its output to go low. The low output from U636b is applied
to the plus input of comparator U636c causing its output to go low. The low output of U636e is
coupled through inverters U634a and U634b to the RESET input of the microprocessor to reset it. The
RESET signal from inverter U634b is also applied to gate U633b causing its output to go low and reset
& counters U631, U632a and U632b.
Calibration Reset. The microprocessor is reset prior to each calibration routine to insure that valid
measurements are obtained. When calibration is requested, the microprocessor stops toggling the OTL
signal at U6II pin 19, and waits for the circuit to time out as follows:
cause its output to go low and reset count U631, U632a and U632b. After being reset the counters
begin counting the 2 MHz clock signal applied to U631, pin 10. After counting for slightly more than
0.5 seconds the output of counter U632b pin 9 is set high. The high output of U632b is applied to the
minus input of comparator U636b causing its output to go low. The low output from U636b is applied
to the plus input of comparator U636c causing its output to go low. The low output of U636e is
coupled through inverters U634a and U634b to the RESET input of the microprocessor to reset it. The
RESET signal from inverter U634b is also applied to gate U633b causing its output to go low and reset
counters U631, U632a and U632b.
8-55. AC to DC Converter
The AC measurement functions are voltage, current, frequency and period. They can be used in either
an AC coupled mode or a DC coupled mode. For AC current measurements. the input signal is routed
through the same set of shunt resistors used for DC current measurements and the AC voltage drop
across those resistors is measured. A simplified block diagram of the AC section is shown in Figure 8-3.
Notice the circuitry contains two compensated attenuators, two X10 gain stages, an RMS converter, a
es filter and various control circuits. The AC to DC Converter can be separated and explained as follows:
HP 3457A Multimeter
8-27