This chapter covers the basics of using a discrete signal on a single logic I/O port to sense and control things in the physical world. It also touches on topics such as buffers, logic-level translation, and current sink and sourcing considerations.
A discrete interface involves a single signal, typically binary in nature. This is probably the most common, and useful, type of interface encountered in digital electronics. It is also the simplest. It is either true or false, on or off. The microwave oven is powered on, or it isn’t. The key is in the ignition, or it isn’t. The infrared motion sensor is either active, or it isn’t. And so on, and so forth. The opposite of discrete is analog, the realm of indefinite variable values. Chapter 13 covers analog interface concepts and components.
The term discrete comes from the realm of programmable logic controllers (PLCs) used in industrial control systems, and it has an advantage over a more general term such as digital in that it specifically implies a single signal or circuit intended for use as an interface to some external device. The term digital could mean anything from a single circuit carrying one bit of information between ICs to the multiple signals found in a parallel digital bus. Of course, the term digital can also mean an interface that responds to or generates binary signals for use with external devices, but I’ve elected to use the term discrete to make the distinction clear.