An advanced electronic postage meter system is described, which is built around a micro computer set. The micro computer set is of LSI design, and comprises a single chip central processor unit (CPU) which performs all control and data processing functions. Auxiliary to the CPU are ROM's which store the program of the postage meter system; RAM's which provide the system with a working memory; and Shift Registers which expand the I/O capacity of the system and provide multiplexing capability. The postage meter system comprises componentry such as a non-volatile memory for postage accounting purposes; a display for visually monitoring the functions of the system; a keyboard for instructing the system; and a modified postage meter with motorized setting means for printing postage upon pieces of mail. These peripheral devices communicate with the micro computer set through ports, and means are provided to expand port capabilities for these peripheral devices.