Abstract: Units peripheral to a data processor and individually selectable by unique address signals to perform functions are arranged in sets providing functions of hard wired apparatus such as data modems, units in each set providing functions identical to corresponding units in other sets. The performance of the functions is controlled by a memory unit which designates which modem set is to be selected and by a common program routine which instructs the processor to apply, to an address bus, address codes defining corresponding units without specifying the individual unit. A logic circuit converts the address codes to the unique address signals of the corresponding unit in the designated modem set. At the end of the routine the processor changes the designation in the memory unit to another modem set.