figures are designated with the same reference numerals fig1 illustrates a simple signature analyzer of the type known in the prior art with this analyzer an automatic test pattern generator atg 10 applies a sequence of test patterns stimuli to the circuit under test cut 12 the response to each test pattern is passed to a linear feedback shift register lfsr 14 which changes its state upon receipt of each response the state of the lfsr 14 is reflected by a 34 signature 34 namely the distinct bit pattern appearing at the parallel output lines 16 of the lfsr this signature is passed to a digital display device 18 which displays a unique set of characters defined by each different signature both the cut 12 and the lfsr 14 are brought to their initial state at the beginning of the test by a pulse on an input line 20 each stimulus and response is clocked into the cut and the lfsr respectively by a clock pulse on and input line 22 in practice the operator of the signature analyzer shown in fig1 notes the displayed signature only at the conclusion of each test sequence he then compares this signature with the correct signature noted in a test manual to determine if the cut is operating properly the manual comparison step may be eliminated by replacing the display device 18 with a comparator.