The aim of this report is to demonstrate the application of a microprocessor for data communication on minicomputers. Two types of data communication units interchangeable between Siemens 310, 320 and 330 process computers, both types running with the same software, are described. In order to enable a universal utility, the functions of the data communication unit have been divided into two modules, the universal programmable module and the specific data communication module. The former is used as a module to connect simple periphery devices to the central processor. The universal programmable module contains the microprocessor. The specific data communication module is a module for specific adaption of the universal programmable module to data communication duties (to manage the data communication procedures according to the appropriate ISO-Standards).