PURPOSE:To easily bracket a fault by connecting a MODEM of a center side and plural MODEMs of a remote side, sending out a testing data to the MODEMs of the remote side by five kinds of menus, and deciding a received data. CONSTITUTION:The following menus (M) are performed by executing an operation 21 from a center side. By M (1), one MODEM of a remote side is selected manually, a carrier is turned on and off from a transmitting part 19 through a carrier controlling circuit 16, an instruction generating circuit 17, and an SD switching circuit 18, the error check 14 of a data from a receiving part 20, the decision 15 of an instruction, and a timer measurement 13 are executed, and whether the abnormality exists or not is checked. By M (2), the remote side is brought to polling dynamically and successively. By M (3), when the abnormality is detected, the polling is stopped. By M (4), the only remote side MODEM designated from the center side is checked repeatedly. By M (5), it is stopped forcibly to send out the carrier of the remote side, from the center side, the stop is released successively, and the modem when is always sending out the carrier is detected. In this way, a faulty part can be detected easily.
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