also connected via a resistance 140 to the input of a schmitt trigger 142 the output of the schmitt trigger 142 is connected via a resistance 144 and a capacitance 146 to the earth the input of the schmitt trigger 142 is also connected to a node intermediate the resistance 144 and the capacitance 146 the can driver 112 has its canh and canl terminals connected respectively to the canh and canl terminals of the can bus 300 the cangnd terminal is connected to the datum voltage u201c 0 u201d it will be appreciated that the interface circuit 100 is made up of two similar complementary interface circuits 150 elements 102 110 amp 114 124 and 152 elements 126 148 which respectively interface pulses for transmission on the bus 300 and pulses received from the bus 300 pulses relative to earth potential received from the txduc terminal of the ecu 200 are differentiated by the capacitor 102 to produce positive going and negative going spikes corresponding to the pulses 39 positive going and negative going transitions are passed by the transformer 106 the resulting signal consisting of positive going and negative going spikes separated by a dc level at the output winding of the transformer 106 is applied to the schmitt trigger 110 in which a positive going spike at its input causes its upper trigger level to be crossed resulting in the output of the schmitt.