referring to the block diagram of fig1 an acquisition system 10 and a display system 12 are generally interconnected by a microprocessor system including a comparison logic and control unit 14 and its associated bus 16 , forming a digital oscilloscope . analog signals are applied via an input terminal 20 to an input conditioning circuit and analog - to - digital converter ( adc ) 22 . these are conventional circuits and typically include input buffering , attenuation , preamplification , and triggering to condition the signal to suitable levels and frame starting point for conversion to n - bit parallel digital data by the adc . the adc suitably may be of the type that is sample - clocked or strobed to provide the digital data at a predetermined conversion rate . the digital data is stored in an acquisition memory 24 under the control of an address counter ( not shown ) which operates in synchronism with the sample clock that drives the adc . the acquisition memory 24 typically has capacity to store one waveform , or one screen - width frame of information . the display system 12 includes a display waveform memory 30 , a display clock and address counter 32 , a digital - to - analog converter ( dac ) and vertical amplifier system 34 , a dac and horizontal amplifier system 36 , and a display device 38 . the named blocks contain conventional circuits commonly used in digital oscilloscope display systems . the display waveform memory is loaded with data representing a waveform by the microprocessor 14 via the bus 16 . the stored display data is clocked out of memory 30 under control of the display clock and address counter 32 , converted to analog signals and displayed on the vertical axis of the display device . likewise , the address data produced by the address counter portion of block 32 are converted to analog values to drive the display along the horizontal axis of the display device 38 . the display device 38 for this particular system is a cathode - ray tube , although other graphic display media , such as flat - panel display devices , may be used . a z - axis amplifier 40 is also provided to turn the crt display beam on and off in accordance with the waveform information to be displayed . two additional waveform memories are provided on the bus 16 -- a reference waveform memory 42 and a changed - waveform memory 44 . upon command manually asserted by an operator or in accordance with programmed instructions , the comparison logic and control unit 14 transfers the contents of the acquisition memory 24 to the reference waveform memory 42 thereby to store a reference waveform . thereafter , new waveforms may be acquired by the acquisition system 10 , perhaps on a repetitive basis at predetermined time intervals in the conventional manner . all of the successive waveforms are triggered or initiated in the same manner and therefore are expected to be substantially identical to the reference waveform . the comparison logic and control unit 14 compares each newly acquired waveform stored in the acquisition memory 24 with the reference waveform in memory 44 . the comparison logic may be implemented in hardware using digital comparators , latches , and gate circuits , or the comparison logic may be implemented in software . both techniques are well known in the art . when any deviation from the reference waveform is detected in a newly - acquired waveform , that waveform containing the deviation automatically is stored in the changed waveform memory 44 by the microprocessor portion of comparison logic and control unit 14 . the detected deviation may be in the form of a glitch , missing pulse , ringing aberrations , transients , or any other change in the waveform , even a complete change or the loss of the waveform . several display modes are possible because the contents of memories 24 , 42 , and 44 are equally accessible and may be transferred to the display waveform memory 30 . one such display mode contemplated is to display both the reference and changed waveforms , alternating between the two in a manner similar to the conventional alternate mode associated with analog oscilloscopes . in this mode , the reference waveform is displayed by itself on the display device 38 until the changed waveform is detected , at which time the alternate mode begins . appropriate level - shifting dc voltage may be added to either or both waveforms at the vertical amplifier stage to cause vertical displacement on the viewing screens , separating the waveforms for easy visual comparison . the fact that two waveforms are so displayed indicates that a change has occurred and has been stored , so that it is not necessary that the user monitor the display watching for the change , or even be present when the change occurs . another display mode contemplated is to display the changed waveform with the changed portion intensified . in fig2 a , a reference waveform 50 is shown using the leading edge of a pulse as an example . the microprocessor portion of comparison logic and control unit 14 may generate a guard band , shown by the dashed lines 52 , substantially coincident with the reference waveform to establish the limits of permissible deviation . the guard band may be programmed to accommodate any desired or anticipated characteristic . in fig2 b , the waveform 50 is shown shifted to the right . the comparison logic portion of unit 14 detects a change in waveform because a portion of the waveform is outside the guard band . not only is the changed waveform stored in memory 44 , but the addresses of the deviant waveform portion are stored in an intensified zone random - access memory ( ram ) 56 , shown connected between bus 16 and z - axis amplifier 40 in fig1 . the changed waveform is transferred to display waveform memory 30 , and is then clocked out for display by the display clock and address counter 32 . when the address counter reaches the addresses stored in ram 56 , logic signals are generated and applied to the z - axis amplifier 40 , which in turn increases the bias current of the cathode - ray tube 38 , intensifying the changed part of the waveform . the intensified zone is shown by the heavy line 58 in fig1 c . in addition to the waveform display modes discussed above , the microprocessor portion of comparison logic and control unit 14 may be programmed to perform any of several well - known analysis techniques , such as signature analysis , on the waveforms to detect change . it will therefore be appreciated that the aforementioned and other desirable objects have been achieved ; however , it should be noted that the particular embodiment of the invention which is shown and described herein is intended as merely illustrative and not as restrictive of the invention .