Electronic circuits are divided into two categories, digital and analog. An analog quantity varies continuously and digital quantities only have discrete values. Digital systems can process, store, and transmit data more efficiently. But it can only assign discrete values. Many systems use a mix of analog and digital signals. For example, a CD player accepts digital data from the CD drive and converts it to an analog signal.
Positive Logic and Negative logic
In the binary system, the digits 1 and 0 are called binary digits. To represent these two bits, we use two different voltage levels. These voltages are called logic levels.
Generally higher voltage HIGH represents 1 and low voltage LOW represents 0. This is called positive logic.
Another system that represents 1 with LOW and 0 with HIGH is called negative logic.
In a practical digital circuit, however HIGH and LOW can be any voltage between a specified minimum and maximum voltages.