34 as to those signal events in the skew compare logic unit of fig1 a the amplitude detector 507 a fig2 a is a schmitt trigger which is provided with a threshold voltage such that no output will be registered unless the threshold voltage is met or exceeded this insures that when the 34 skew 34 is considered beyond the level of acceptability only then will the schmitt trigger operate to produce an output signal which will generate an error condition to the latch 508 a further details and operative features of the data comparator module are explained and shown in connection with the following fig1 b 2a 2b 3 and 4 a series of comparator cards as elements 1 through 14 fig4 are used to provide the various functions shown in fig1 and 3 the comparator cards provide the following functions a they provide a self test function to check the adjusted skew window with the generation of a 34 pass delay 34 test circuit and a 34 fail delay 34 test circuit as shown in fig1 b the pass test circuit sets up a skew timing which must pass all of the skew compare circuits in order to validate the system operation the fail delay test circuit sets up a skew period which every circuit sees as a failure this is a system whereby assurance is made that the skew compare logic unit of fig1 a is functioning accurately this particular set of pass fail tests is executed in the self test mode function for every single.