a system for transmitting and receiving serial data over a pair of differential lines so that relatively large differences in ground potential will not affect the integrity of the transmitted data . each differential line has a first switch for pulling the line high and a second switch for pulling the line low . logical gates hold all switches off to allow the lines to float in response to a first control signal . a second control signal enables the logical gates to pull the first differential line up and the second differential line down whenever true data is to be sent and for pulling the second differential line up and the first differential line down whenever data is true . associated with each switch is a protective diode and cold filament lamp for protecting the respective switch and insuring system operation even if large swings in ground potential occur . a differential amplifier has one input resistively coupled to the first differential line and a second input resistively coupled to the second differential line . a pair of matched resistors are coupled between the inputs of the differential amplifier and a reference point so that the values of the resistors establish the rail - to - rail operating range of the system . various resistors at the input of the amplifier enable the data to be determined by differences in the currents in the first and second differential lines and not by the difference between a signal and local ground .