Abstract: A fiber optic extender system in which messages are transferred by fiber optic lines between remotely located electronic units. The electronic units (e.g., transformer-isolated) have three signal levels: positive voltage, negative voltage, and zero voltage. Because the fiber optic system can conveniently handle only two signal levels--on and off--potentially serious communication problems are avoided by generating and transmitting a short end-of-message pulse whenever the transmitting electronic unit is at zero voltage level. This end-of-message pulse is detected at the remote electronic unit, and the line is cleared for the next message.