An I/O expansion bus for microcomputers that is independent of processor or board type is specified. Each expansion interface supports up to 16 8-bit I/O ports directly. Enhanced addressing capability is available using slave processors or FIFO devices. In addition, each expansion interface may optionally support a DMA channel capable of data rates up to 2 16-bit Mwords/sec. These features are supported for both 8- and 16-bit data paths. The specification has been prepared for those users who intend to design or evaluate products that will be compatible with the bus. For this purpose, functional, electrical, and mechanical specification is covered in detail. The intent of the specification is to guarantee compatibility between baseboards and expansion modules while not restricting the actual designs any more than necessary.