PURPOSE: To provide a universal architecture which can perform the serial communication in a simple circuit constitution by securing connection via a common bus among a 1st port which receives the parallel data from a host computer, a 2nd port which supplies the parallel data to a converter means and a 3rd port which supplies the serial data to the outside.

CONSTITUTION: A host computer/communication device system contains three ports 56, 57 and 58. A CPU bus is connected to a parallel transmitting interface 76 via the 1st port 56. A bus 78 is connected to the interface 76 and an SCC block 77. The data written into the port 56 are directly transferred to a 2nd port, i.e., a parallel port. The 2nd port 57 is connected to a microprocessor and the data transferred to the port 57 are converted into the serial data. The block 77 having a serial communication function transfers the converted data to a local microprocessor via the 3rd port 58.