if signal levels are in fact present and if such signal levels are approximately of equal level indicating proper operation and if at different levels or if one level is not present indicating improper operation of one of the receivers r 1 or r 2 the portion of the dmgi logic shown in fig3 further indicates which of the two receivers r 1 or r 2 is faulty similarly electrical signals generated in the two transmitters t 1 l and t 2 are supplied to the dmgi logic 150 via leads 144 and 142 which will compare such outputs to determine if either of the transmitters t 1 or t 2 is faulty and which is the faulty one in the event of the failure of either of receivers r 1 or r 2 or either of transmitters t 1 or t 2 a message will be supplied to the remaining operating transmitter of transmitters t 1 and t 2 indicating which one is faulty such signal will be supplied to an assigned microprocessor at one of the nodes on the loop not specifically indicated in the drawings for diagnostic and maintenance purposes it is to be understood that receivers r 1 and r 2 fig2 receive optical signals through four port star coupler 164 from fo bus input terminal 166 and translates such optical signals into electrical signals which are supplied to and through dmgi logic 150 and to processor 110 for various types of processing as will be described in more detail later herein the signals supplied.