HP 3457A Service Manual page 188

Microprocessor Inoperative Reset. Under certain conditions it is possible for the microprocessor to
"lock-up" or become non-operational. During normal operation, the microprocessor continually toggles
the INOP signal at USI0, pin 19. If the TNOP fails to toggle, the processor is reset by the Watch Dog
circuit as follows:
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 TNOP
signal at U510 pin 19, and waits for the circuit to time out as follows:
A continually high or low INOP signal applied to capacitor C505 allows the input of inverter US91b to
go low.
The high output of U591b and the normally
high RESET signal applied to the inputs of gate
U598c cause its output to go low and reset counters U514 and USI5.
After being reset the counters
begin counting the 2 MHz clock signal applied to U514, pin 10. .After counting for slightly more than
0.5 seconds the output of counter U515 pin 9 is set high. The high output of USI5 is applied to the
minus input of comparator
U519a causing its output to go low.
The low output from U519a is applied
to the plus input of comparator U519d
causing its output to go low. The
low output of US19d is
coupled through buffer U594d to the RESET input of the microprocessor to reset it, The RESET signal
from buffer U594d is also applied to gate US98c causing its output to go low and reset counters U5I4
and USIS.
8-49. Main Controller (03457-66511)
Beginning with serial number 2538A02954, 3457A's were shipped with a new main controller board, the
03457-66511.
This board works similar to the original main controller, the 03457-66501, and is com-
pletely backwards compatible.
The following sections describe operation of the new main controller in
detail.
Refer to schematic Al 1/1 for the following discussion.
The Main Control Assembly manages the overall operation of the instrument. This includes managing
the measurement process, performing mathematical calculations, storing data, and communicating with
the front panel and with other instruments connected to the interface bus.
HP 3457A Multimeter
8-23