an interface driver circuit , for use in a programmable energy load controller system , comprises a pair of optically - coupled input sections , respectively receiving data and a special signal of digital nature and a first voltage polarity . a buffer amplifier section is coupled to the data input circuit for driving a twisted - wire transmission line , coupled to the paralleled inputs of a plurality of receivers , with logic signals alternating essentially between ground potential and the first voltage at the first polarity ; a second amplifier section is enabled by the special signal input section and impresses a voltage of the remaining polarity across the interface driver circuit output when commanded at the special signal input , regardless of the state of the data signal at the first input .