a data communication system comprises a plurality of terminal units having different addresses individually and a main control assembly connected to the terminal units by transmission lines for controlling the terminal units . the terminal units include a first group of terminal units and second group of terminal units . the main control assembly controls the first group of terminal units according to a series of instructions for controlling the units in a predetermined sequence . when executing one instruction , the main control assembly performs data communication with only the one specific terminal unit relevant to the instruction . concomitantly with the communication with the specific terminal unit , the main control assembly scans and communicates with the second group of terminal units only during periods that specified conditions exist . in addition , means are provided for communicating malfunction information between the main control assembly and the terminal units .