PURPOSE: To diagnose a DMA operation through a system, and to detect an abnormal state exactly and quickly, by transmitting data stored in a master station side in advance, to a main storage device by a DMA mode, or the like.

CONSTITUTION: Data stored in a master station 3 side in advance is transferred to a main storage device 5 by a DMA mode. For instance, control information to a slave station 11 is generated by a central operating unit CPU 1, is stored in the main storage device 5, and at the necessary time point, this control information is updated. Subsequently, a fetch request of contents of the main storage device 5 is sent out to a controller II 14 periodically from the master station 3, the information is read out from a stored address designated by the main storage device 5 through an extending bus 13, a controllerI12 and a memory bus 6, is inputted to the master station 3, and is turned back to a transmission processor 9 and is outputted. Subsequently, from the slave station 11, displaying information is sent out to the controller II 14 through the transmission processor 9 and the master station 3, a store address of the main storage device 5 is designated through the controllerI12, etc., and this displaying information is written.