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Understanding Grounds

The movement of the electrical current along a conductor requires a path for the current to return to its source. In early telegraph systems and even modern power-transmission systems, the earth provides a return path and, hypothetically, produces an electrical reference point of absolute zero. Figure 3 depicts this type of ground.

Figure 3 Power-transmission system.

Many electronic circuits use an actual conductor as a return path. This type of ground is referred to as a signal ground. Electronic devices might also contain a third form called chassis ground, or protective ground. In any event, ground still remains the reference point from which most electrical signals are measured. For troubleshooting computer components, measurements referenced to ground can be made from the system unit's chassis.

The other measurement reference is the signal ground point on the printed circuit board, where the test is being performed. This point is not too difficult to find in a circuit board full of ICs because most DIP-style chips use the highest numbered pin for the positive supply voltage and the last pin on the pin-1 side of the chip as the ground pin (see Figure 4). Some caution should be used with this assumption because not all ICs use this pin for ground. If you examine a number of ICs and connectors on the board, however, you should be able to trace the ground foil and use it as a reference.

Figure 4 Grounds on IC chips.

Grounding is an important aspect of limiting EMI in computer systems. Left unchecked, EMI can distort images on the video display, interfere with commercial communication equipment (such as radios and televisions), and corrupt data on floppy disks. In addition, EMI can cause signal deterioration and loss caused by improper cable routing. If a signal cable is bundled with a power cord, radiation from the power cord could be induced into the signal cable, affecting the signals that pass through it. Good grounding routes the induced EMI signals away from logic circuitry and toward ground potential, preventing it from disrupting normal operations. Unlike ESD, which is destructive, the effects of EMI can be corrected without damage.

Test Tip

Remember that ESD is destructive and EMI is not.

Because the computer system is connected to an actual earth ground, it should always be turned off and disconnected from the wall outlet during electrical storms. This includes the computer and all its peripherals. The electrical pathway through the computer equipment can be inviting to lightning on its way to earth ground. The extremely high electrical potential of a lightning strike is more than any computer can withstand.

Test Tip

Know the best protection for a computer system during an electrical storm.

This article has described the danger and causes of electrostatic discharge and provided information on how to eliminate them. By reviewing this information and paying special attention to the "Test Tip" boxes, you should be well prepared for questions on Objective 3.2 in the exam's Core Hardware module.

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