Abstract: A serial data communication loop system includes single input-output controllers and/or dual message transmission with a novel adaptor unit for converting from a single to dual message frame and/or from a dual to single message frame. The message frames are multiple bit binary coded signals. The adaptor unit includes first and second converting sections, the first converting from a single input to dual output message frames and the second from a pair of identical messages to a single message output. The first section includes an amplifier connected to a polarity and level sensor and to a time delay which activates a disable timer to the amplifier and a strobe timer to operate a pair of transmitters to transmit the dual message frames. The second section includes a pair of receivers connected to a logic selector in combination with individual monitors which encode the status at the receivers to normally transmit from one receiver and automatically change to the second if the first fails.