Input devices of a digital computer:

Input devices are used to read the instructions and data to be processed and output devices display the results obtained after executing the program. Keyboard, Mouse and Scanner are examples of input devices, whereas Monitor, Printer and Plotter are examples of output devices. Memory is used as a working storage for temporarily storing the data and intermediate results generated during program execution. Computers use two kinds of memories: primary & secondary. The primary memory is often referred to as RAM in everyday language. It is a read/write memory used to store both the program and data. Since RAM is volatile, computers also use a second level of memory-  secondary memory-  to permanently store the contents. Hard Disk is the non-removable secondary storage device which stores virtually everything on the machine. Computers also use other removable secondary memories like CD-ROMs, Magnetic tapes and recently Flash Drives to permanently take backup of the data onto Hard Disk or to transfer data from one machine to another.         

A more practical description of a digital computer can be given by describing the major units and their interconnections for a simple personal computer (PC). If you open the CPU cabinet of your PC, you will notice that it contains a printed circuit board on which a number of devices are plugged in. This printed circuit board is often called the mother board. All other major components of the computer are either plugged in directly to this mother board or connected through a bunch of wires. CPU, RAM and Device Cards are plugged in various slots of the mother board. Devices like Hard Disk, Floppy Drive, CDROM Drive, which are attached to the CPU cabinet, are connected through wire ribbons. The mother board has printed circuitry which allows all these components to communicate with each other. CPU cabinet also houses a power supply unit which provides power to all the components of the computer system. On the back end of the CPU cabinet, you can notice a number of connection slots. These slots are used to connect various input/output devices such as keyboard, mouse, printer, scanner, to the computer.