A communication control method for double ring networks including a plurality of transmission stations, each configured with a pair of communication ports for communications to be implemented bi-directionally, and mutually connected by said pairs of communication ports in the double ring networks through a transmission line to implement intercommunications between the transmission stations, characterizednin a state that transmission frames are kept from continuing circulation in the double ring networks by two adjacent terminal stations, with the initialization completed, under a condition that one or more transmission stations are compliant with a specifically undefined transmission line control system for avoiding collisions between transmission frames on the double ring networks, to send specific information-containing one or more transmission frames to be periodically sent synchronization frames, and transmission frames to be sporadically sent, whereinthe respective transmission station comprises:ring repeat request frame reception detecting means for detecting reception of ring repeat request frames containing specific control information, respectively;own address setting means for setting information for identification of own station;adjacent station address setting means for setting information for identification of transmission stations adjacent to communication ports, respectively;address match detecting means for detecting a match by comparison between identification information of own station and destination information in a received ring repeat request frame from a reception output of the ring repeat request frame reception detecting means;blocked port resetting means for responding to a match output of the address match detecting means by changing a ring repeat request reception port having received said ring repeat request frame to an unblocked state;ring repeat request reception responding means for responding to a mismatch output of the address match detecting means by reading identification information of an adjacent transmission station corresponding to the ring repeat request reception port from the adjacent station address setting means, to have as a destination to be addressed, and sending a ring repeat request frame generated with the destination, through the ring repeat request reception port, immediately in time for completion of reception of the ring repeat request frame; andring repeat request responding acknowledgment means for detecting by checking for reception of an ring repeat request frame addressing own station as a destination, within a preset time, following said sending said ring repeat request frame,as an anomaly has occurred in a transmission line or at a transmission station on a route from a transmission station (referred herein to a synchronizing station) for sending synchronization frames, this transmission station being inclusive, with terminal stations inclusive, for a respective transmission station on way from a location of occurrence of anomaly to the terminal stations, by a step of detecting an anomalous state by no-synchronization detecting means and reception anomaly detecting means in response to the occurrence of anomaly, andfor a respective transmission station adjacent to the location of occurrence of anomaly on routes from the location of occurrence of anomaly to the synchronizing station, by a step of detecting an anomalous state by the reception anomaly detecting means, whereinas-having-detected communication ports have their status turned to blocked states, respectively, and between said terminal stations being two, for one terminal station in a subsequent position on a route from the synchronizing station to the location of occurrence of anomaly, by a step of determining at this terminal station (referred herein to a synchronization absence detecting terminal station) an anomaly as having occurred on the route to the synchronizing station, from a detection output of the anomalous state by synchronization absence detecting means and a no-synchronization detection output of the synchronization absence detecting means, andfor the other terminal station (referred herein to a synchronization normal terminal station), which continuous receiving synchronization frames normally, by a step of determining a route from this synchronization normal terminal station to the synchronizing station as having a maintained normal communication function,for the synchronization absence detecting terminal station, bya step of responding to a detection of occurrence of anomaly of a state of synchronization absence from the synchronization absence detecting means and the no-synchronization detecting means, by immediately sending ring repeat request frames addressing the synchronization normal terminal station as a destination, through the communication ports, anda step of waiting, at the ring repeat request responding acknowledgment means, for a response from a transmission station adjacent on an opposite side to the synchronization normal terminal station,for the synchronization normal terminal station, by a step of responding to reception of a ring repeat request frame from the synchronization absence detecting terminal station, as own station is a destination, by changing a ring repeat request reception port that has been in a blocked state as a terminal station, to an unblocked state by said blocked port resetting means,as the terminal station status is cancelled, for a respective transmission station functioning normally on a way from the synchronization absence detecting terminal station to the location of occurrence of anomaly, by a step of responding to reception of a ring repeat request frame from the synchronization absence detecting terminal station, as being not a destination, by sending a ring repeat request frame having as a destination an adjacent transmission station on the way to the synchronization absence detecting terminal station, by said ring repeat request reception responding means, through a ring repeat request reception port, immediately after a completion of reception of the ring repeat request frame,for the synchronization absence detecting terminal station, by a step of responding to reception in time of a ring repeat request frame addressing own station from an adjacent station on the way to the synchronizing station, by the ring repeat request responding acknowledgment means, by changing a communication port that has been in a blocked state since the time of anomaly detection, to an unblocked state, anda step of following this changing to change, after lapse of a time the ring repeat request responding acknowledgment means has preset, also the other communication port that has been blocked since before as a terminal station, to an unblocked state, thereby canceling the terminal station status,for the respective transmission station functioning normally on the way from the synchronization absence detecting terminal station to the location of occurrence of anomaly, by a step of responding to acknowledgment of reception of a ring repeat request frame addressing own station from the adjacent station on the way to the synchronizing station, by the ring repeat request responding acknowledgment means, by changing a communication port that has been in a blocked state since the time of anomaly detection, to an unblocked state,for a respective transmission station adjacent to the location of occurrence of anomaly on way to the synchronization absence detecting terminal stations, by a step of following absence of reception of a ring repeat request frame addressing own station over lapse of a preset time, by the ring repeat request responding acknowledgment means, to hold a communication port as it is blocked, to function as a new terminal station, andfor occurrence of anomaly at a location adjacent to a synchronization absence detecting terminal station, likewise followed by absence of reception of a ring repeat request frame addressing own station over lapse of a preset time, by the ring repeat request responding acknowledgment means, by a step of holding a communication port that has been in a blocked state since the time of anomaly detection, as it is blocked, and in thatthe other communication port that has been in a blocked state since before as a terminal station is changed to an unblocked state, and as new terminal stations, a terminal station on a way from the anomaly occurrence location to the synchronizing station, and a terminal station on an opposite way from the anomaly occurrence location are changed to be new double ring network's terminal stations, whereby a network of the ring form is re-configured upon occurrence of communication anomaly.
