Frequency hw subtest, Threshold hw subtest, Reset hw subtest – Fluke 900 User Manual
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FLUKE 900 SERVICE MANUAL
Selftest
If this test fails it does not set one of the data bits but rather sets the shifting error flag.
Frequency HW subtest
The frequency hardware tested is on the High Speed Board. It can be configured in
several different ways (the setup for this is shown in the MODE box on the schematic), aU
of which are independently tested.
1.
Read a fiequency. A 25MHz clock is generated and the frequency is measured.
2.
Read the period. Pin #2 is selected through the frequency mux (this hardware is
tested in a different selftest). It is setup to read the period and then the CPU
generates a cycle of fixed duration on this line using the puUup registers on the
Interface Buffer. The period displayed should be approximately 990 us.
3.
Read the high time and low time. These are done in the same way as the period test
except the times are displayed. Low time should be approximately 815 us and high
time 175 us.
Threshold HW subtest
This subtest programs a value of threshold and reads it back to verify that it is within an
acceptable range. The variation between the measured value and the value programmed
(in mV) is displayed on the screen. The long test goes through all possible settings
between 0 and 5 volts and the short test only goes in steps of 1 volt
After the threshold setting has been tested the threshold is calibrated to compensate for
input offsets in the comparators. The puUups are used to set the actual threshold on the
input to the comparator to 4.16 and 0 volts and then the threshold is adjusted to see what
offset added to the amount programmed (4.2 and 0 volts) will be closest to the actual
threshold of the comparators. If the adjustment exceeds an allowable amount the
calibration part of this test win fail.
Reset HW subtest
For this subtest the reset relays are set so that power for reset comes from 5 volts and the
output is fed into the analog mux.
a) reset = high: must be > 4 volts
b) reset = low: must be < 1 volt
c) reset = off: must be < 4 volts & > 1 volt
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