1. Apparatus for selecting, storing and displaying a qualified subset of the states occurring in a collection of digital signals, the apparatus comprising: control means for designating from among the collection of digital signals a qualification state and a trigger state; input means coupled to receive the collection of digital signals for performing signal conditioning thereon according to preselected thresholds to produce a collection of conditioned signals; qualification state detection means coupled to the collection of conditioned signals and to the control means for producing a qualifier signal upon the occurrence of the designated qualification state in the collection of conditioned signals; trigger state detection means coupled to the collection of conditioned signals and to the control means for producing a trigger signal upon the occurrence of the designated trigger state in the collection of conditioned signals; storage means coupled to the collection of conditioned signals, the qualifier signal and to the trigger signal for storing those states in the collection of conditioned signals producing the qualifier signal, until a preselected number of such storage operations occur subsequent to the occurrence of the trigger signal; and display means coupled to the storage means for displaying the states stored therein.