Removing the effects of unknown test values from compacted test responses

An apparatus used in testing a circuit-under-test, comprising:

• a compactor configured to receive test response vectors output from scan chains in the circuit-under-test;
• and
• a logic network coupled to the compactor and configured to receive the compacted test response vectors output from the compactor,
• the logic network being controllable by one or more control signals and configured to logically combine selected test values of the compacted test response vectors in response to the control signals.

A circuit, comprising:

• a circuit-under-test that is part of the circuit;
• a compactor coupled to the circuit-under-test and configured to compact test responses from the circuit-under-test;
• and
• a filter that removes the effects of one or more unknown test values in the test responses compacted by the compactor, the filter being coupled to outputs of the compactor.