PURPOSE: To obtain a data transmitter/receiver capable of efficient data communication in a system using a microcomputer.

CONSTITUTION: At the time of data communication in the system using the microcomputer, data is transferred from a communication device 2 to a memory 4 not by way of CPU 3 but by using a DMA controller 1 and when deciding a previously-set protocol finish condition, a finish condition deiciding circuit 6 gives a control signal for finishing DMA to the DMA controller. Besides, it is made possible to set the protocol finish condition from CPU to the finish condition decidign circuit. Thus, the burden of CPU is reduced to be appropriate for increasing the communication speed. Besides, the setting of the protocol finish condition is attained, reception through the use of the DMA controller is attained concerning data of an optional protocol.