Abstract: In a highly modular terminal, pluggable functional units communicate optically through a closed chamber. Some functional units are logically integrated into the terminal structure while being physically remote. In a supermarket system, bar code scanners and produce scales are examples of physically remote functional units. A circuit for interfacing the remote units to the closed chamber uses a broadcast/concentrator mode of operation. A single optical detector broadcasts the extant chamber signals to all of the remote functional units. Signals from the remote units are logically ORed or concentrated to provide a drive signal for a single optical emitter capable of injecting optical energy into the chamber.