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COMMENTS
The indication of an output short is a very positive indicator of a problem. Tri-state or open-
collector circuit design can be checked easily against a known good board or, as in all cases,
the circuit can be investigated further by checking the components being driven by the output
and a choice being made to remove a component and test independently.
EXERCISE 20 : SIGNAL DETECTION
ACTION
Press START on the IC Tester.
Press CHANGING SIGNAL switch and observe SIG indication on Pin 5.
Note the status will not show pass due to the alternating nature of the signal
DESCRIPTION
If an IC is being functionally tested on a board that has a clock oscillator running, and the
clock or fast signal derived from the clock is present on one or more pins of that IC, then a
SIGNAL message may appear in yellow on those pins. This message is an indication that a
changing signal has been detected on that pin. The test may be affected in such a way that a
good IC could FAIL.
COMMENTS
Changing signals on the board under test are one of the biggest causes of spurious test
failures. Even if they do not cause a particular test to FAIL, they may cause different circuit
conditions that can cause problems when comparing results between different boards using
test sequence or IC Live Comparison modes especially if comparing voltages.
In this test there are spurious bad comparisons even thought the SIGNAL is correctly
identified. There are MIDHI and MIDLO indications because of the voltage varying whilst the
measurement is being carried out therefore the saved result has to be only one of the values.
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