Abstract: A device is disclosed which converts a digital signal or bit stream into a digital signature repesentative of the digital signal by means of a feedback shift register. The apparatus may be used to identify and characterize digital signals at various test points in an apparatus for testing purposes. Signatures for digital signals from properly operating circuits can be recorded in a variety of fashions for later comparison with signatures of digital signals from circuits under test. The comparison of the signatures enables a person using the apparatus to determine whether the circuit under test is operating properly and, if it is not, to locate the fault in many instances. The apparatus may also be used to examine digital signals to enable identification of transient errors.