JP2833910B

PURPOSE:To reduce the test for a multiplexer/demultiplexer circuit in a communication controller and to make the test efficient. CONSTITUTION:A storage device 5 stores transmission data from line adaptors 1-4 by one channel. A comparator circuit 10 compares the transmission data of one channel with reception channel data stored in the storage device. When a loopback instruction is received from a host equipment, a transmission frame control section 8 at first stores channel data from a 1st line adaptor to the storage device 5. Then the channel data from the first line adaptor are used as in-frame channel '0' data and channel data stored in the storage device as in-frame channel '1' and subsequent data are used to compose a transmission frame. In the execution of loopback, as for channel data after the in-frame channel 1 data in the reception/transmission frame with equal content as the transmission frame, the comparator circuit 10 compares them with the channel data in the storage device 5 and the result of comparison is outputted to the host equipment through a signal line 18.