Plant control system

Claim 1

A loop communication system comprising a plurality of controller stations, at least one display station including means for displaying the status of each of said controller stations, and a loop transmission channel through which said controller stations and said display station are connected in a loop to permit data transmission between said controller and display stations while giving a transmission right to each of said controller and display stations in sequential order so that the station to which the transmission right is given is allowed to send out into the transmission channel data originating in that status, wherein each of said controller and display stations comprises a transmission interface circuit and a self-diagnostic circuit connected to said loop transmission channel, said transmission interface circuit in each station including means for effecting data transmission with the other stations by way of said transmission channel, and said self-diagnostic circuit in each station including means for diagnosing the operation of its station for the presence or absence of a malfunction and for disconnecting its station from said transmission channel when it diagnoses that its station is disabled, wherein said transmission channel includes loop transmission lines arranged in duplex, said transmission interface circuit in each station including means for receiving data from and applying data to one of said transmission lines for effecting data transmission between its station and the other stations by way of said one transmission line, and said self-diagnostic circuit in each station including means for selecting said one of said transmission lines and for disconnecting its station from said selected transmission line when it diagnoses that its station is disabled, and wherein said transmission interface circuit includes first selector means for selecting data received through one of said two loop transmission lines, register means for registering the received data selected by said first selector means, second selector means connected to said first selector means and said register means for selecting the data registered in said register means when the transmission right for data transmission over said selected transmission line has been received and for selecting the data supplied from said first selector means when the transmission right has not been received, transmission control circuit means for receiving the data selected by said second selector means and for processing said received data thereby producing processed data, and third selector means for selecting either the processed data or the data transmitted by the selected one of said transmission lines according to a signal determining a transmission mode and for supplying the data selected by said third selector means to the selected one of said two transmission lines; and said self-diagnostic circuit in each station includes first circuit means for receiving the data transmitted through said selected one of said transmission lines and for producing a first signal when said received data meets predetermined requirements indicating that the received data is normal, second circuit means for receiving the data transmitted to said selected one of said transmission lines and for producing a second signal when said received data meets predetermined requirements indicating that the received data is normal, and third circuit means for receiving a signal originating in the station having that third circuit means and producing a third signal when said received signal meets predetermined requirements indicating that the received signal is normal, and an AND gate connected to receive said first, second and third signals and producing a signal to select one of said transmission lines in the presence of all of said first, second and third signals.