be seen with a sinusoidal discovery tone of 1 cycle per 2 microseconds 500 khz from pwrup the loop back signal turns off in less than 1 microsecond after pwrup goes high fig6 is a plot 96 of the loop back discovery tone the discovery tone may preferably be a sinusoidal signal of less than a few megahertz in frequency a sinusoidal signal is not absolutely required but is preferred because it is less likely to cause spurious emissions a signal of less than a few megahertz in frequency will easily propagate with insignificant voltage loss on twisted pair wire to the well known ethernet point to point maximum connection requirement of 140 meters fig7 is a plot of the differential loop back discovery tone plot 98 corresponds to the voltage at node 100 and plot 102 corresponds to the voltage at node 104 fig7 illustrates the voltage at node 100 and 104 where no electro static discharge esd diodes are present in phy 106 fig8 is an electrical schematic diagram of a conventional esd protection circuit 108 for a conventional phy 106 esd diodes d 1 d 2 d 3 and d 4 clip voltage on lines rx u2212 and rx to avoid damage to sensitive electronic circuits inside phy 106 the result is typically that instead of the 2 5 volt peak to peak swings of fig7 the measured voltage at node 100 corresponds to plot 110 of fig9 and the measured voltage at node 104 corresponds to plot 112 of fig9 which show peak to peak voltage swings of only about 1 4 volts an alternative specific embodiment of the present invention is shown in fig1 a and 10b which.