The Maplin Electronic Circuits Handbook, 2nd Edition

176 COMPUTER INTERFACING

voltage may fall to around 1.5 V when a port line is sourcing an appreciable

current, Darlington transistors are much preferred.

Adding a 'user port'

In some cases a microcomputer may be limited in its port capability (or

an existing parallel port may be dedicated to some other peripheral

hardware (such as a printer). It then becomes necessary to add some extra

I/O capability. This may be achieved using either a fully-blown pro-

grammable parallel I/O device or, alternatively, a single byte (or less) of

input or output may be added quite easily.

In either case it will, of course, be necessary to have a reasonably intimat ...