Patent ID: 7852790

Claim:
A communication master station applied to a PLC system, in which one PLC device having a communication facility and one or more I/O terminal devices having the communication facilities are connected with a bus type network, the PLC device having the communication facility is set to a communication master station while each of the I/O terminal devices having the communication facilities is set to a communication slave station, and, in an operation state, the corresponding communication slave station repeats a series of operations where a response frame is returned to the communication master station for frame transmission from the communication master station and thereby various pieces of data are exchanged between the communication master station and each communication slave station, the communication master station comprising: a master plurality determination part which detects existence of plural communication master stations on the network is performed in advance of a link establishment process and/or in parallel with the link establishment process in a procedure of making transition to the operation state through the link establishment process after startup; a first master plurality avoidance part which makes a transition to a predetermined master plurality avoidance process without making the transition to the link establishment process when it is determined that plural communication master stations exist by a master plurality determination process performed in advance of the link establishment process; and a second master plurality avoidance part which makes the transition to the predetermined master plurality avoidance process without making the transition to the operation state when it is determined that plural communication master stations exist by the master plurality determination process performed in parallel with the link establishment process, wherein, in the master plurality determination process perfoii ied in parallel with the link establishment process, the master plurality is determined based on a fact that a reception of a response frame is not confirmed from any communication slave stations in a connection frame confirmation process for the link establishment, a polling frame being transmitted to each of all the connectable communication slave stations to store presence or absence of the reception of the response frame from the communication slave station in the connection frame confirmation process for link establishment.