As it’s name implies, input devices are devices which can send signals to the CPU. That means, the device we use to send or enter a signal or data to CPU are called input device, such as; keyboard, mouse, microphone etc.
Why keyboard is an input device?
As it is known that the use of the input devices are to send signals to the CPU, keyboard was one of the first peripherals to be used with computer to input data. The use of keyboard is to input text, numbers and many other signal commands into the computer system using keys in different key groups of keyboard.
The way through a keyboard works proves that keyboard is an input device. After we press any key, a tiny chip called keyboard controller places a code into the keyboard buffer which is a temporary memory of the keyboard controller. Then keyboard controller sends a signal to the system software that something happened at the keyboard. The system software reads the code which was saved in the keyboard buffer and makes a signal to the CPU.
This process says that, by pressing the keys of the keyboard an user send data or signal to the computer. As the device which is used to send signal to the computer is known as input device, that is why keyboard is an input device.