AC Electrics - Logic Gates — Page 268, Lesson 278

I want to walk you through the fundamentals of logic gates, starting with what they actually are and why they matter in aircraft electrical systems. Logic gates—often just called gates—are a type of fundamental function performed by computers and related equipment. Think of them as the basic building blocks that make digital decision-making possible. Inside a computer, a single integrated circuit, or IC, contains several gate circuits. Each gate can have several inputs, but it must have only one output. That's a fixed rule: multiple inputs, one output. There are six commonly used logic gates that you'll encounter: the AND gate, the OR gate, the INVERT gate, the NOR gate, the NAND gate, and the EXCLUSIVE OR gate. The name of each gate tells you exactly what function it performs, so the name is your first clue to understanding its behaviour. Now, let's talk about the language these gates speak. Logic gates are binary in nature. That means the inputs and the outputs are always in one of two possible states, expressed by the digital notation '1' or '0'. But you'll see these states referred to in several different ways depending on the context. A '1' can also mean: on, true, high (often written as just 'H'), closed, or engaged. A '0' can mean: off, false, low (written as 'L'), open, or disengaged. So when you see any of those terms, they're all pointing to the same binary concept. To make sense of how a gate behaves, we use something called a truth table. A truth table is a systematic way of displaying binary data. It shows the relationship between a logic gate's inputs and its output. For troubleshooting purposes—and this is very practical for you as a technician—the truth table data is often reviewed to determine what the correct output signal should be for a given set of inputs. If you know the inputs, the truth table tells you what output to expect. Each logic gate also has a symbol with a specific shape. These symbols are designed to point in a given direction: the inputs are always listed on the left of the symbol, and the output is always on the right. That's a standard convention you'll see everywhere. Since logic gates operate using digital data, all input and output signals are 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. So when you see a '1', think of a positive voltage being present; when you see a '0', think of ground or zero volts. Now, here's an important distinction: positive logic versus negative logic. As I just mentioned, logic circuit input and output signals consist of two distinct levels, often called 'binary 1' and 'binary 0'. But the actual voltage levels required to achieve a 'binary 1' or 'binary 0' can vary between circuits. That's where the logic convention comes in. 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 could be both positive, 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'. So it's all about which voltage is more positive. Most digital systems employ positive logic throughout the entire computer and related component circuitry, so that's the convention we'll be using in the examples that follow. Let me show you what this looks like visually. That figure shows the symbol and truth table for an AND gate, which is the first of the six gates we'll be covering. The symbol has inputs on the left and the output on the right, and the truth table systematically lists all possible input combinations and the resulting output. We'll dive into each gate's specific behaviour next.