Computers can also be divided into three categories depending upon their instruction and form of input data that they accept and process. These are:

1. Analog Computers
2. Digital Computers
3. Hybrid Computers

1. Analog Computers

The word "Analog" means continuously varying in quantity. The analog computers accept input data in continuous form and output is obtained in the form of graphs. It means that these computers accept input and give output in the form of analog signals. The output is measured on a scale. The voltage, current, sound, speed, temperature, pressure etc. values are examples of analog data. These values continuously increase and decrease. The analog computers are used to measure the continuous values. The thermometer is an example of analog device because it measures continuously the length of a mercury column. Another example of analog computer is the analog clock because it measures the time by means of the distance continuously covered by the needle around a dial. Similarlym speedometer, tire-pressure gauge are also examples of analog devices.

The analog computers have low memory size and have fewer functions. These are very fast in processing but output return is not very accurate. These are used in industrial units to control various processes and also used in different fields of engineering.

2. Digital Computers

The word "Digital" means discrete. It refers to binary system, which consists of only two digits, i.e. 0 and 1. Digital data consists of binary data represented by OFF (low) and ON (high) electrical pulses. These pulses are increased and decreased in discontinuous form rather than in continuous form.

In digital computers, quantities are counted rather than measured. A digital computer operates by counting numbers or digits and gives output in digital form. A digital computer represents the data in digital signals 0 and 1 and then processes it using arithmetic and logical operations. Examples of digital devices are calculators, personal computers, digital watches, digital thermometers etc. Today most of the computers used in offices and homes are digital computers.

The main features of the computers are:

• Give accurate result.
• Having high speed of data processing.
• Can store large amount of data.
• Easy of program and are general purpose in use.
• Consume low energy.

3. Hybrid Computers

The hybrid computers have best features of both analog and digital computers. These computers contain both the digital and analog components. In hybrid computers, the users can process both the continuous (analog) and discrete (digital) data. These are special purpose computers. These are very fast and accurate. These are used in scientific fields. In hospitals, these are used to wathc patient's health condition in ICU (Intensive Care Unit). These are also used in telemetry, spaceships, missiles etc.

Differentiate between Analog and Digital Computers

ANALOG	DIGITAL
Accept input data in continuous form and output is measured on a scale. It may have some errors in output. Have low internal memory. Have fewer functions. It is used only in scientific, industrial and medical fields. It is costly. It is not easily programmed.	Accept input data in digital form and output is received in digital form. Output is accurate. Have large internal memory. Have large number of functions. It is general purpose in use. It is low in cost. It is easily programmed.