Apparatus for a signalling system, includes a microprocessor operative to process data words and command words within first and second message intervals. During the first message interval, the microprocessor operates to generate and apply data words in succession to an output terminal and to receive and test command words received in succession at an input terminal. During the second message interval, the microprocessor operates to receive and test data words received in succession at the input terminal and to generate and apply command words in succession to the output terminal. Each command word includes flag data specifying the word as a command word and command data representing either a "send" command or a "retransmit" command. Each data word includes flag data specifying the word as a data word. The proper receipt and acceptance of a "send" command word in the first message interval results in a data word being applied to the output terminal, and the proper receipt and acceptance of a "retransmit" command word in the first message interval results in the previous data word being reapplied to the output terminal. The proper receipt and acceptance of a data word in the second message interval results in a "send" command word being applied to the output terminal; otherwise, a "retransmit" command word is applied to the output terminal.