A system and method that enables testing of circuitry using an externally generated signature. An external tester is arranged external to a device under test (DUT). Such external tester is operable to input test data to the DUT, receive output data from the DUT, and generate a signature for at least a portion of such received output data. The external tester compares the generated signature with an expected signature to determine whether the DUT is functioning as expected. If the generated signature fails to match an expected signature, then error data can be written to an error map log. Preferably, further interaction with the DUT is not required after detecting that a generated signature fails to match an expected signature in order to perform such error evaluation. Thus, error evaluation can be performed concurrently with testing of the DUT. Mask data may be stored in a compressed form, and decompressed and used for masking certain non-deterministic output bits in generating the signature.