Network unit

A network unit, in a control system including a plurality of programmable logic controllers and a plurality of control networks respectively allocated with a network number, that can create diagnosis information, the network unit indicates a state of a control network connected with the network unit, the network unit comprising:

• a diagnosis-information transmission request unit that transmits a diagnosis-information transmission request comprising a header part that includes a destination network number indicating a respective control network from among the plurality of control networks and a destination station number which represents the network unit which connects a respective programmable logic controller from among the plurality of programmable logic controllers to the respective control network wherein the destination stations number is a predetermined number of a value other than any station number assigned to any network unit;
• a routing control unit that:
• stores routing information indicating correspondence between a destination control network and identification information on a network unit that is a relay station for the destination control network, and
• relay-controls the diagnosis-information transmission request received from the diagnosis-information transmission request unit in the network unit, or another network unit connected to the same programmable logic controller as the network unit, or a control network connected to the network unit, in accordance with the routing information;
• and
• a diagnosis-information creation unit that:
• creates the diagnosis information in response to receiving the diagnosis-information transmission request having a network number of the control network connected to the network unit as the destination network number, and
• transmits the created diagnosis information to a sender of the diagnosis-information transmission request,
• wherein the network unit further comprises a diagnosis-information management unit that, in response to determining that there is a network number that has not transmitted the diagnosis-information transmission request in the network numbers included in an acquired list, repeats transmission of the diagnosis-information transmission request to the network numbers included in the acquired list until there is no network number that has not transmitted the diagnosis-information transmission request.