Parallel I/O is the most used I/O device in small microcomputer applications. A parallel I/O
means that there are a number of static input or output bits. These outputs can be used to
drive some lights, to switch some relays, to drive a display and so on. The input bits can be
used to read information from switches etc.
A simple parallel I/O can be done by a simple register component like a 74LS373. There are
also some special parallel I/O components available, like the PIO 8255 or the Z80-PIO. With
components the user can switch every bit as an input or output, so these components are
very flexible for many applications.