Abstract: Balancing of a "transmit" line and a "receive" line, which make up a half-duplex serial transmission line, is accomplished, where a first optical isolator is utilized to drive an electrical current signal over the "transmit" line and a second optical isolator is utilized to receive electrical current signals from the "receive" line, by adding a third optical isolator to the circuit. The third optical isolator is in an on condition when the first optical isolator is in an on condition and is in an off condition when the first optical isolator is in an off condition. The third optical isolator is used to make the "receive" and "transmit" lines both see a "signal ground", at the ends thereof connected to the first and second optical isolators, when current is flowing in the "transmit" line.