however if the value is correct then it has been determined that the tristate elements 18 are tristating correctly and a pass code is generated as given in block 56 determining logically the value at point 26 and at the c element 24 can be accomplished using the same methods that are traditionally used with a standard automated test pattern generator if a failure is detected the root cause can be investigated by loading the keeper element 20 with the value it held at the beginning of the failing pattern and loading combinations of patterns into the d elements 14 to determine the failing mechanism for example if the keeper element 20 is at the same state as all but one of the d elements 14 and the pattern fails then the single d element 14 with the different state is not tristating properly alternately many different combinations of d element 14 patterns can be used to isolate the d element 14 values that actually cause a fail condition for example if a value of zero that is loaded into the keeper element 20 failed it would be anticipated that one of the tristated drivers 18 failed to tristate or if some of the tristate devices 18 were leaky possibly a failure would be seen when three out of five for example of the d elements 14 were set to one in some instances it might not even matter which three of the five were set to one thus being able to test combinations of data.