Distributed control apparatus

A distributed control system in which a plurality of PLCs is connected together via active and backup transmission lines which are opposite in transmission direction and which have a ring-type topology, wherein each of the PLCs comprises:

• regular direction diagnosis command sending means for sending a diagnosis command to a PLC located downstream on the active transmission line via the active transmission line whose transmission direction is the regular direction;
• and
• reverse direction diagnosis command sending means for sending a diagnosis command to a PLC located downstream on the backup transmission line via the backup transmission line whose transmission direction is opposite to the regular direction,
• wherein the PLCs includes a master PLC which administers system diagnosis and at least one slave PLC;
• the regular direction diagnosis command sending means and the reverse direction diagnosis command sending means of the master PLC send the diagnosis command to the slave PLC as a trigger for system diagnosis;
• and
• upon receipt of the diagnosis command, the regular direction diagnosis command sending means and the reverse direction diagnosis command sending means of the slave PLC send the diagnosis command to another slave PLC or the master PLC.