Abstract: A method and implementation is described by which I/O input and output circuitry of a CMOS chip are measured without the need to probe the chip. Output driver transistors are used to provide marginal voltages to test input circuits, and the output driver transistors are segmented into portions where a first portion is used to provide a representative “on” current, which is coupled to a test bus that is further connected to a current comparator circuit contained within the chip. Both leakage and “on” current of the driver transistors is measured using segmented driver transistors. The output of the current comparator circuit is connected to a test scan register or to a test output from which test results are obtained digitally. The testing techniques are also applicable for other semiconductor devices.