Recall that there are two basic types of electronic signals—analog and digital. A digital signal is one that varies in discrete steps. Unlike an analog signal, which varies continuously, a digital signal has two levels or states. The signal switches or changes abruptly from one state to the other.
Figure 5.1 shows a DC digital signal that switches between two known levels such as zero volts or close to it (<0.1 volt) or 0 V and +3.3 V. The positive voltage can be anything between about 1 volt and 12 volts with 3.3 and 5 being the most common.
Digital signals with two discrete levels are also referred to asbinary signals. Binary means two—two states or two discrete levels of voltage.
Humans use the decimal number system that represents quantities with digits 0 through 9. However, digital equipment and computers do not. Internally, digital equipment processes binary data.