Abstract: A parallel data bus tester is particularly applicable to SCSI type buses, and uses a plurality of comparator circuits to simultaneously examine the voltages present on the conductors of a bus cable. For twisted pair type conductors, the tester uses a high reference voltage, which is compared to the high voltage conductor of each pair, and a low reference voltage, which is compared to the low voltage conductor of each pair. The reference voltages are adjustable, and allow each reference voltage to be swept through the expected voltage range for its respective conductors. As the level of a reference voltage crosses that of a conductor voltage being examined, a comparator that receives the reference voltage and conductor voltage changes output state. This output is used to drive a corresponding LED assigned to the conductor in question. Thus, as the reference voltage, which is compared to each of its corresponding conductor voltages (i.e.