A data transmission system for transferring data between a plurality of processors (200A - 200G, 200Z, 2000) and between the processors and an input/output unit (303) through a common bus (301) has linkage units (300A - 300G, 300Z, 300L) between the processors and the bus and an address controller (302) to manage the bus. Each of the linkage units has a two-port random access memory (42A - 42G) for storing data necessary for the processor. The processor processes the data stored in the memory and writes a result of the processing in an output area of the memory. The processing of the processors and the data transfer through the bus are essentially separated and carried out independently.