Patent ID: 6931443

Claim:
A data communication system comprising: a main station; plural substations, each of which is connected to said main station by a common transmission line; a data transmitter which transmits data address to all of said plural substations, provided in said main station; plural data receivers which receive said data, provided in said plural substations respectively; a message transmitter which transmits a query message addressed to all of said plural substations after said data are transmitted, to inquire whether reception was normal, provided in said main station; and plural response transmitters which transmit a carrier as a response message to said query message, only when said data could not be received normally, provided in said plural substations respectively; wherein, whenever said main station receives a certain signal wave after transmitting said query message, said main station judges the signal wave said carrier transmitted from one or more of said plural substations and retransmits said data addressed to all of said plural substations and wherein when said main station detects no signal during a certain time period, the main station transmits the next transmission data.