Solid-State Devices Used in Industrial Logic Circuits: SOLID-STATE DEVICES USED FOR LOGIC: Early Examples of Logic Gates

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Logic gates became widely used in the late 1950s and early 1960s soon after the transistors were mass-produced on integrated circuits (ICs). Since the ICs were made on a single chip of silicon, they also became known as chips. The outline of the device is called dual in-line package (DIP) because it has two rows of pins that allow it to maintain a low profile when it's mounted on a circuit board. The integrated circuits provide a variety of logic devices in multiple input AND and OR gates as well as NAND and NOR gates. Since these devices all used similar voltages of approximately 5 volts, they became known as the TTL family (transistor- transistor logic). TTL chips also provide more than one logic gate on an IC. One common IC has four AND gates mounted on it (QUAD AND gate).

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The TTL family of chips does have some limitations when larger groups of gates are used together. Then the speed of the logic operation is slowed down, so the inverting-type gates (NAND and NOR) may be substituted because they use fewer transistors and other components. The inverter gates also use less power. The main drawback with inverted logic is that it seems more difficult to follow because humans have been taught to think from “front-to-back” instead of “back wards.” It's important to understand that the same logic functions can be achieved by either AND and OR gates or with NAND and NOR gates.

TTL gates are still in use today, but a large variety of similar devices has been designed to help overcome some of the problems associated with them. For instance, a complete logic family of complementary metal oxide semiconductors (CMOS) devices have been designed that are fast acting and use less power. The CMOS devices also have some inherent problems such as their susceptibility to damage due to static charges. Since the CMOS devices require very little power to operate, they can also be severely damaged by small amounts of stray power. The thin metal oxide insulation layer at the gates of these devices can also be damaged by static discharges from equipment or humans. Because of this problem, you must adhere to strict procedures for grounding yourself when you work on CMOS circuits.

Other logic circuit gates and components are available as low-voltage technology (LVT) that uses BiCMOS. This family of devices is designed to operate at 3.3 volts instead of the higher TTL voltage levels of 5 volts. Another family of logic devices is called low-voltage CMOS (LVC). which operates at voltage levels of 2.7 to 3.6 volts. These devices use less than 24 mA for drive current and can react at speeds of less than 6.5 ns. These logic devices have evolved to provide reliable, low-power, high-speed logic circuits.

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Monday, August 25, 2008 1:48