Abstract: A time domain reflectometer (TDR) for making impedance measurements on a powered transmission line cable system. The TDR is particularly useful for measuring impedance discontinuities on a powered Ethernet system. A negative test pulse is provided to ensure that any positive reflections do not exceed the upper voltage limit of the transceivers. The TDR selectively applies a negative bias to the transmission line cable below the collision detection threshold in order to disable all transceivers and prevent network interference with the TDR measurements. The negative DC bias also provides additional headroom for positive pulse reflections.