a unique method and structure is provided for testing high voltage equipment with great accuracy of voltage levels to be measured , repeatability of measured levels from one piece of test equipment to the next , no need for recalibration of test equipment , and sufficiently low current during testing that the test equipment can be powered by the same supply that powers the device under test . the device of this invention can be used to measure not only logical one and logical zero voltage levels of the device under test while under specific loads but can also be used to meausre transition time from one logic state to another . the circuit of this invention provides an output signal which can have three states reflecting a high logic level from the device under test , a low logic level from the device under test , and an intermediate level indicating that the device under test is in transition from one logic level to another or has failed the test .