The basic functional units of computer are made of electronics circuit and it works with electrical signal. We provide input to the computer in form of electrical signal and get the output in form of electrical signal.
There are two basic types of electrical signals, namely, analog and digital. The analog signals are continuous in nature and digital signals are discrete in nature.
The electronic device that works with continuous signals is known as analog device and the electronic device that works with discrete signals is known as digital device. In present days most of the computers are digital in nature and we will deal with Digital Computer in this course.
Computer is a digital device, which works on two levels of signal. We say these two levels of signal as High and Low. The High-level signal basically corresponds to some high-level signal (say 5 Volt or 12 Volt) and Low-level signal basically corresponds to Low-level signal (say 0 Volt). This is one convention, which is known as positive logic. There are others convention also like negative logic.
Since Computer is a digital electronic device, we have to deal with two kinds of electrical signals. But while designing a new computer system or understanding the working principle of computer, it is always difficult to write or work with 0V or 5V.