Chapter

17

AC Electrics - Logic Gates

An Introduction to Logic Gates . . . . . . . . . . . .	. .	. .	. .	. .	. .		.263
Binary Logic . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	263
Truth Tables . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	263
Gate Symbols . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	263
Positive and Negative Logic . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	263
The ‘AND’ Gate . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.264
The ‘OR’ Gate . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	265
The ‘INVERT’ or ‘NOT’ Gate . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	266
The ‘NAND’ Gate . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	267
The ‘NOR’ Gate . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.268
The ‘EXCLUSIVE OR’ Gate . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.269
Questions . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.270
Answers . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	274

261

17 AC Electrics -Logic Gates

Gates Logic - Electrics AC 17

262

AC Electrics - Logic Gates 17

An Introduction to Logic Gates

Logic gates, or gates, are a type of fundamental function performed by computers and related equipment. A single integrated circuit (IC) within a computer contains several gate circuits. Each gate may have several inputs and must have only one output.

There are six commonly used logic gates: the ‘AND’, the ‘OR’, the ‘INVERT’, the ‘NOR’, the ‘NAND’ and the ‘EXCLUSIVE OR’. The name of each gate represents the function it performs.

Binary Logic

Logic gates are of a binary nature, i.e. the inputs and the outputs are in one of two states expressed by the digital notation ‘1’ or ‘0’. Other corresponding expressions are also frequently used, they are:

•‘1’ - on; true; high (H); closed; engaged

•‘0’ - off; false; low (L); open; disengaged

Truth Tables

Truth Tables are a systematic means of displaying binary data. Truth tables illustrate the relationship between a logic gate’s inputs and outputs. This type of data display can be used to describe the operation of a gate. For troubleshooting purposes, the truth table data is often reviewed in order to determine the correct output signal for a given set of inputs.

Gate Symbols

Each logic gate has a symbol of a specific shape. The symbols are designated to “point” in a given direction, that is, the inputs are always listed on the left of the symbol and the outputs on the right.

Since logic gates operate using digital data, all input and output signals will be made up of ‘1’s or ‘0’s. Typically the symbol ‘1’ represents ‘ON’ or voltage positive, and the symbol ‘0’ represents ‘OFF’, or voltage negative. Voltage negative is often referred to as zero voltage or the circuit’s ground.

Positive and Negative Logic

As stated earlier, logic circuit input and output signals consist of two distinct levels. These levels are often referred to as ‘binary 1’ and ‘binary 0’. The actual voltage levels required to achieve a ‘binary 1’ or ‘binary 0’ may vary between circuits.

•If positive logic is used in the digital circuit, a ‘binary 1’ equals a high voltage level and a ‘binary 0’ equals a low voltage level. The actual voltage values may be either both positive or both negative, or one positive and one negative. The only stipulation for positive logic is that a ‘binary 1’ is created by a greater positive voltage than a ‘binary 0’ . Each signal represents the greater positive voltage value as a ‘binary 1’, and therefore the following examples employ the positive logic concept. Most digital systems employ positive logic throughout the entire computer and related component circuitry.

AC Electrics - Logic Gates 17

263

17 AC Electrics -Logic Gates

•The negative logic concept defines ‘binary 1’ as the lower voltage value and ‘binary 0’ as the higher voltage value (more positive). Although less popular, negative logic is used in some systems in order to meet certain design parameters.

The ‘AND’ Gate

The ‘AND’ gate is used to represent a situation where all inputs to the gate must be ‘1’ (on) to produce a ‘1’ (on) output. To be an ‘AND’ gate, input No. 1 and input No. 2 and input No. 3 etc, must be ‘1’ to produce a ‘1’ output. If any input is a ‘0’ (off), the output will be ‘0’ (off). The symbol and the truth table for a two-input AND gate are illustrated in Figure 17.1.

Gates Logic - Electrics AC 17

Figure 17.1 The symbol and truth table for an ‘AND’ gate

A simple ‘AND’ circuit may also be represented by two switches in series used to turn on a light as shown in Figure 17.1. If both switches (inputs) are ‘1’ (on), the light will turn ‘1’ (on). If either switch is ‘0’ (off), the light will be ‘0’ (off).

The ‘AND’ gate is sometimes called an ‘ALL or NOTHING’ gate.

264