For constructing reliable, high-grade digital transmission networks, exact failure or degradation detection in digital transmission equipment, such as regenerative repeaters, is important. A new measurement for eye opening estimation of equalised signals, called the transmission waveform deflection-type margin (TWDM) measuring method, is proposed. The principles of the method are described and the measurement accuracy in relation to measurement time required is analysed. Experimental results are presented and the measurement method shown to have a wide range of applications.