PURPOSE: To attain data communication with high accuracy and reliability by reducing a reception interval of data by circulating data in a ring and providing plural communication data transmission and reception states between communication equipments at the same time.

CONSTITUTION: Various data are communicated in a ring by using a DMA function among three microcomputers or over (four in figure as 4, 6, 8, 10). Each of the microcomputers 4, 6, 8, 10 is provided with a 2nd DMA access means which receives data to each memory by DMA transfer and reads the memory content or adds data and relays and gives at least part of the memory content to a next stage. Furthermore, each of the microcomputers 4, 6, 8, 10 is provided with an initial data transmission means sending data twice or over at the start of data communication. Thus, two communication data or over are circulated in the system at all times, then reception frequency of communication data is higher and the accuracy and reliability of data communication are enhanced.