PURPOSE:To produce the data path efficiently, by checking the state of line of the respective channel and the state of the controller at line connection. CONSTITUTION:The data transmission channel control SVC station transmits the data to the remote station RS1 and checks whether or not this data is correctly received at the RS1 station and returned to SVC again. In case of the process input and output stations PRS1-PRSn having the process input and output channel controllers P10C1-P10Cn, if the flag is absent in the status in the return data, the same processing is made to the next RS2. If the flag is present, data is transmitted to P10C1 after being selected to the process input and output channel. P10C1 checks this data and returns the answer to SVC when normal. If no response, P10C1 is disconnected and the processing of the next RSn is given.