Computers

Definition: The term computer has been borrowed from compute that means to calculate.A computer is an electro mechanical device which has capacity to accept, store, process and retrieve data, according to the user requirements.

Characteristics of a computer:

The following are the characteristics of a typical computer:

Speed: Present day computers operate at a very high speed. A computer can perform several million instructions in one second. For example, it can add or multiply 2,000,000 numbers in a second. There are several different types of computers and they all have different speeds ranging from high to very high.The speed of computers is specified in MIP (Million instructions per second). The speed of computer is closely related to the amount of data it must process. The term volume and frequency are often used to describe the amount of data handled by a computer system. Volume may represent the overall quantity of data to be processed.Frequency described as low often a specific data item is used in processing.

Accuracy: Computers perform with a very high degree of consistent accuracy.Errors may occur due to a program logic error.These are human errors. Hardware errors are usually detected by the computer system itself.The probability of computer error is small and often traceable to faulty data. Computers rarely make mistakes and can accurately perform all kinds of complex computations.

Reliability: Computer systems are particularly adept at repetitive tasks.They do not take sick days and coffee breaks and they seldom complain. Computer systems are widely accepted because of their exceptional reliability. Unlike most humans they are capable of operating under most adverse conditions for extended periods of time without showing any signal of fatigue. Computers consistently provide the same accurate results under all operating conditions.

Memory Capabilities: Computer systems have total and instant recall of data and an almost unlimited capacity to store these data.A typical mainframe computer system will have many billions of characters, thousands of graphic images, stored and available for instant recall.

Versatility: The same computer can be used for various applications. For instance, a PC can be used to prepare a letter, prepare the balance sheet of company, store a database of employee, send/receive fax message etc. For a computer to perform a new job, all it needs is a program.

One simple test you can perform on a power supply is to check the output voltages. This shows whether a power supply is operating correctly and whether the output voltages are within the correct tolerance range. Note that you must measure all voltages with the power supply connected to a proper load, which usually means testing while the power supply is still installed in the system and connected to the motherboard and peripheral devices.

You need a simple digital multimeter (DMM) or digital volt-ohm meter (DVOM) to perform voltage and resistance checks on electronic circuits (see Figure 3.17). You should use only a DMM instead of the older needle-type multimeters because the older meters work by injecting 9v into the circuit when measuring resistance, which damages most computer circuits.

Figure 3.17 A typical DMM.

A DMM uses a much smaller voltage (usually 1.5v) when making resistance measurements, which is safe for electronic equipment. You can get a good DMM with many features from several sources. I prefer the small, pocket-size meters for computer work because they are easy to carry around.

Some features to look for in a good DMM are as follows:

• Pocket size. This is self-explanatory, but small meters are available that have many, if not all, of the features of larger ones. The elaborate features found on some of the larger meters are not really necessary for computer work.

• Overload protection. This means that if you plug the meter into a voltage or current beyond the meter's capability to measure, the meter protects itself from damage. Cheaper meters lack this protection and can easily be damaged by reading current or voltage values that are too high.

• Autoranging. This means that the meter automatically selects the proper voltage or resistance range when making measurements. This is preferable to the manual range selection; however, really good meters offer both autoranging capability and a manual range override.

• Detachable probe leads. The leads easily can be damaged, and sometimes a variety of differently shaped probes are required for different tests. Cheaper meters have the leads permanently attached, which means you cannot easily replace them. Look for a meter with detachable leads that plug into the meter.

• Audible continuity test. Although you can use the ohm scale for testing continuity (0ohms indicates continuity), a continuity test function causes the meter to produce a beep noise when continuity exists between the meter test leads. By using the sound, you quickly can test cable assemblies and other items for continuity. After you use this feature, you will never want to use the ohms display for this purpose again.

• Automatic power off. These meters run on batteries, and the batteries can easily be worn down if the meter is accidentally left on. Good meters have an automatic shutoff that turns off the unit when it senses no readings for a predetermined period of time.

• Automatic display hold. This feature enables you to hold the last stable reading on the display even after the reading is taken. This is especially useful if you are trying to work in a difficult-to-reach area single-handedly.

• Minimum and maximum trap. This feature enables the meter to trap the lowest and highest readings in memory and hold them for later display, which is especially useful if you have readings that are fluctuating too quickly to see on the display.

Although you can get a basic pocket DMM for as little as $20, one with all these features is priced in the $100-$200 range. Radio Shack carries some nice inexpensive units, and you can purchase the high-end models from electronics supply houses, such as Newark or Digi-Key.

To measure voltages on a system that is operating, you must use a technique called back probing on the connectors (see Figure 3.18). You cannot disconnect any of the connectors while the system is running, so you must measure with everything connected. Nearly all the connectors you need to probe have openings in the back where the wires enter the connector. The meter probes are narrow enough to fit into the connector alongside the wire and make contact with the metal terminal inside. The technique is called back probing because you are probing the connector from the back. You must use this back-probing technique to perform virtually all the following measurements.

Figure 3.18 Back probing the power supply connectors.

To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on AT, Baby-AT, and LPX supplies; pin 8 on the ATX-type connector) for +3v to +6v of power. If the measurement is not within this range, the system never sees the Power_Good signal and therefore does not start or run properly. In most cases, the power supply is bad and must be replaced.

Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors. If you are measuring voltages for testing purposes, any reading within 10% of the specified voltage is considered acceptable, although most manufacturers of high-quality power supplies specify a tighter 5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of the rating, except for the 3.3v current, which must be within 4%. The following table shows the voltage ranges within these tolerances.

Loose Tolerance

Tight Tolerance

Desired Voltage

Min. (-10%)

Max. (+8%)

Min. (-5%)

Max. (+5%)

+3.3V

2.97V

3.63V

3.135

3.465

+/-5.0V

4.5V

5.4V

4.75

5.25

+/-12.0V

10.8V

12.9V

11.4

12.6

The Power_Good signal has tolerances that are different from the other voltages, although it is nominally +5v in most systems. The trigger point for Power_Good is about +2.4v, but most systems require the signal voltage to be within the tolerances listed here:

Signal

Minimum

Maximum

Power_Good (+5V)

3.0V

6.0V

Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth noting that any and all power supply tests and measurements must be made with the power supply properly loaded, which usually means it must be installed in a system and the system must be running.

You can use several types of specialized test gear to test power supplies more effectively. Because the power supply is one of the most failure-prone items in PCs today, you should have these specialized items if you service many PC systems.

One of the greatest additions to my toolbox is a digital infrared thermometer. They also are called noncontact thermometers because they measure by sensing infrared energy without having to touch the item they are reading. This enables me to make instant spot checks of the temperature of a chip, a board, or the system chassis. They are available from companies such as Raytek (http://www.raytek.com) for under $100. To use these handheld items, you point at an object and then pull the trigger. Within seconds, the display shows a temperature readout. These devices are invaluable in checking to ensure your system is adequately cooled.