JPS6242547B

PURPOSE:To convert a simplex system into a duplex system simply by printed cards added, by preparing two printed cards having the same transmission controlling circuit. CONSTITUTION:When a transmission circuit 16 is connected to a system A, an advance data link ADL controller 4A receives a flag and sets a flip-flop 14A, a direct memory access DMA request signal 8 is outputted from the circuits of system A only, and a DMA permission signal 9 is also inputted to the circuit of system A only for the execution of DMA. A microprocessor MPU1 starts the both transmission controlling circuits of the systems A and B with the same addressing. Since the circuit of the system A enables the gate of a DMA request signal 8A with the flip- flop 14A, only the memory data is transmitted on the bus, and the DMA permission signal 9 is inputted to transmit the memory data to a transmission line 16 through DMA transfer.