📚 Logic Gates | 逻辑门
Logic gates are the fundamental building blocks of digital circuits. In Cambridge IGCSE Computer Science and O Level Electronics, you need to recognise the standard symbols, write truth tables, and understand how simple combinations of gates produce predictable outputs.
逻辑门是数字电路的基本构建模块。在剑桥 IGCSE 计算机科学和 O Level 电子学中，你需要认识标准符号、会写真值表，并理解简单的门电路组合如何产生可预测的输出。
1. What is a Logic Gate? | 什么是逻辑门？
A logic gate is a physical device or electronic circuit that takes one or more binary inputs and produces a single binary output. The output depends only on the current inputs, following a fixed rule. Binary means there are only two possible states: 0 (low voltage, off, false) and 1 (high voltage, on, true).
逻辑门是一种物理器件或电子电路，它接收一个或多个二进制输入并产生一个二进制输出。输出仅取决于当前输入，遵循固定规则。二进制意味着只有两种可能状态：0（低电压、关、假）和 1（高电压、开、真）。
Because logic gates use binary, we can describe every possible behaviour using a truth table. A truth table lists all combinations of inputs and shows the resulting output.
由于逻辑门使用二进制，我们可以用真值表描述所有可能的行为。真值表列出所有输入组合并显示对应的输出。
2. Truth Tables and Binary Logic | 真值表与二值逻辑
In a truth table, each input column represents a binary variable such as A or B, and the right-hand column shows the output Q. For n inputs there are 2ⁿ possible input combinations, so a two-input gate has four rows, while a three-input gate has eight rows.
在真值表中，每个输入列代表一个二进制变量，如 A 或 B，最右侧列显示输出 Q。对于 n 个输入，有 2ⁿ 种可能的输入组合，因此双输入门有 4 行，三输入门有 8 行。
The rows are usually written in binary counting order: 00, 01, 10, 11 for two inputs. This convention makes it easier to compare different gates and to check that no combination is missed.
这些行通常按二进制计数顺序书写：双输入为 00、01、10、11。这种约定便于比较不同的门，并检查是否遗漏了任何组合。
Boolean algebra is the mathematics of binary logic. Each gate has a Boolean expression that shows how the output is calculated from the inputs.
布尔代数是二进制逻辑的数学基础。每个门都有一个布尔表达式，说明输出如何由输入计算得出。
3. The NOT Gate | 非门
The NOT gate has only one input and one output. It inverts the input: if the input is 0, the output becomes 1, and if the input is 1, the output becomes 0. For this reason it is also called an inverter.
非门只有一个输入和一个输出。它对输入取反：如果输入为 0，输出为 1；如果输入为 1，输出为 0。因此它也被称为反相器。
Q = NOT A = A'
| Input A | Output Q |
|---|---|
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