E8257D/67D, E8663D PSG Signal Generators Service Guide 181
Troubleshooting
Self–Test Overview
Self–Test Overview
A group of tests (self–tests) are embedded in the signal generator’s firmware
that can evaluate the condition of the circuitry on many of the assemblies.
Self–tests check hardware performance under controlled conditions, and
compare test results to predetermine limits. If the results fall within the limits
the signal generator passes. If the results of one or more tests fall outside the
limits, the self–test routine evaluates the failures and reports the most
independent failure.
Because self–tests are designed to check signal generator hard ware, the
signal generator does not need to be calibrated for self–tests to pass.
Self–tests are designed with the philosophy that it is better to pass a defective
signal generator than to fail a good one; some failures will pass, rather than be
detected and reported. If the signal generator powers on and self–tests can be
run, always start troubleshooting by running self–tests and resolving reported
problems.
How self–test works:
1. When the signal generator is tested at the factory, self–test limits are
loaded into the A18 CPU memory from devices on each assembly, or from
fixed files. Most assemblies contain limit information, while microcircuit
limits must be loaded.
2. When the program runs, latches and DACs are set to a known state.
3. An analog mux routes the detected dc signal from the selected test node
to the A18 CPU.
4. The A18 CPU compares the detected dc level to the test limit. If the
detected value is within the limits, the A18 CPU reports a pass.
5. If one or more detected values fall outside the limits, the most
independent failure is determined and reported.
Independent failures
An example best illustrates independent failures; in this example, there are
three assemblies in a series: A1, A2, and A3. The output of A1 is the input for
A2, and the output of A2 is the input for A3. If the output of A1 is bad, A2 and
A3 outputs are also bad. If the output of A2 is bad, the output of A3 is also bad.
A file in the signal generator contains this dependency knowledge for the
signal generator to use to identify the most independent failure. If A1, A2, and
A3 self–tests all fail, the signal generator reports A1 as the most independent
failure. If A1 passes but A2 and A3 fail, self–test reports A2 as the most
independent failure. If A2 passes and A3 fails, self–test reports A3 as the most
independent failure. If two independent failures occur, self–test reports only
one failure at a time and reports the second failure after the first reported
failure is repaired.
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