GET SELF_TEST_DESCRIPTION with a "Self Test #" parameter of 0 -- which is defined to be SELF_TEST_OFF -- could return the maximum number of self tests.
Or, as I mentioned before, GET PERFORM_SELFTEST could return two bytes with the maximum in the second byte, just as GET DMX_PERSONALITY does.
A third alternative has ocurred to me: GET PARAMETER_DESCRIPTION can be used to get the minimum and maximum valid values. I had thought of PARAMETER_DESCRIPTION as only being useful for manufacturer-specific PIDs, but it seems pretty useful in this case.
Cheers,
Shaun
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