📚 IGCSE Computer Science: Standard Logic Gate Symbols | 标准逻辑门符号
Logic gates are the fundamental building blocks of all digital systems. In IGCSE Computer Science, you must be able to recognise, draw and interpret the standard symbols for the six common logic gates: NOT, AND, OR, NAND, NOR and XOR.
逻辑门是所有数字系统的基本构件。在IGCSE计算机科学中，你必须能够识别、绘制并解释六种常见逻辑门（非门、与门、或门、与非门、或非门和异或门）的标准符号。
1. Why Logic Gates Matter | 为什么逻辑门很重要
Logic gates are used in processors, memory units and every digital circuit. Each gate performs a simple Boolean operation and produces an output based on one or more binary inputs.
逻辑门用于处理器、存储单元以及每一个数字电路中。每个门执行简单的布尔运算，并基于一个或多个二进制输入产生输出。
The standard symbols allow engineers and students to communicate circuit designs clearly. CIE exams often show a circuit diagram and ask you to write a truth table, a Boolean expression, or to identify the function of the circuit.
标准符号使工程师和学生能够清晰地交流电路设计。CIE考试经常给出电路图，要求你写出真值表、布尔表达式，或判断电路的功能。
2. NOT Gate (Inverter) | 非门
The NOT gate has one input and one output. Its symbol is a triangle pointing to the right, with a small circle (bubble) on the output side. This bubble always represents inversion.
非门有一个输入和一个输出。其符号是一个指向右侧的三角形，输出端带有一个小圆圈（气泡）。这个气泡始终表示取反。
| A | X |
| 0 | 1 |
| 1 | 0 |
X = NOT A
If the input is 0, the output is 1. If the input is 1, the output is 0.
如果输入是0，输出就是1；如果输入是1，输出就是0。
3. AND Gate | 与门
The AND gate has two inputs and one output. Its symbol looks like a D shape: a flat left edge for the inputs and a curved right edge for the output.
与门有两个输入和一个输出。其符号看起来像字母D：左侧平直边用于输入，右侧弧形边用于输出。
| A | B | X |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
X = A AND B (also written X = A · B)
The output is 1 only when both inputs are 1. It is often described as "both inputs must be true".
只有当两个输入都为1时，输出才为1。这常被描述为“两个输入必须都为真”。
4. OR Gate | 或门
The OR gate also has two inputs and one output. Its symbol is a curved shield shape: the left side is curved inward like a crescent, and the right side comes to a point.
或门也有两个输入和一个输出。其符号是弯曲的盾形：左侧像月牙一样向内弯曲，右侧汇成一个尖角。
| A | B | X |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
X = A OR B (also written X = A + B)
The output is 1 when at least one input is 1. If both inputs are 0, the output is 0.
只要至少有一个输入为1，输出就为1。如果两个输入都是0，输出才是0。
5. NAND Gate | 与非门
The NAND gate is an AND gate followed by a NOT gate. Its symbol is exactly the AND-gate D shape with a small bubble on the output line.
与非门是与门后面接一个非门。其符号就是与门的D形，并在输出线上带一个小气泡。
| A | B | X |
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
X = NOT (A AND B)
The output is the exact opposite of the AND gate. It outputs 0 only when both inputs are 1.
其输出与与门完全相反。只有当两个输入都为1时，输出才为0。
6. NOR Gate | 或非门
The NOR gate is an OR gate followed by a NOT gate. Its symbol is the OR-gate curved shape with a bubble on the output line.
或非门是或门后面接一个非门。其符号是或门的弯曲形状，并在输出线上带一个气泡。
| A | B | X |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
X = NOT (A OR B)
The output is 1 only when both inputs are 0. In all other cases the output is 0.
只有当两个输入都为0时，输出才为1。在其他所有情况下，输出都是0。
7. XOR Gate | 异或门
The XOR gate means "exclusive OR". Its symbol is the OR-gate shape with an extra curved line drawn across the input side, between the two inputs.
异或门的意思是“互斥或”。其符号是或门的形状，但在两个输入之间多画了一条弧形线。
| A | B | X |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
X = A XOR B (also written X = A ⊕ B)
The output is 1 when the two inputs are different. This is why XOR is often used in comparison circuits.
当两个输入不同时，输出为1。这就是为什么异或门常用于比较电路中。
8. Reading Combined Circuits | 阅读组合电路
In exams you will often see logic gates joined together. The safest approach is to split the circuit into stages, then work out the output at each stage before writing the final truth table.
在考试中，你经常会看到逻辑门连接在一起。最稳妥的方法是把电路分成几个阶段，先算出每个阶段的输出，再写出最终真值表。
For example, if input A and input B go into an AND gate, and the output of that AND gate goes into a NOT gate, then the final output is NAND. The truth table would include an intermediate column for the AND output.
例如，如果输入A和输入B进入一个与门，然后该与门的输出进入一个非门，那么最终输出就是与非门的功能。真值表应包含一个中间列来记录与门的输出。
| A | B | P = A AND B | X = NOT P |
| 0 | 0 | 0 | 1 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 0 |
Always label intermediate outputs clearly, and use a new column for every gate output. This prevents careless mistakes.
始终清晰地标记中间输出，并为每个门的输出使用一个新列。这可以避免粗心错误。
9. Drawing Logic Gate Symbols Correctly | 正确绘制逻辑门符号
When you draw a logic gate, always place the inputs on the left and the output on the right. Keep all wires horizontal or vertical with clear right-angle connections.
绘制逻辑门时，始终将输入放在左侧，输出放在右侧。保持所有导线水平或垂直，并用清晰的直角连接相交处。
For the AND gate, draw a flat left edge and a smooth curved right edge.
绘制与门时，要画平直的左侧边和光滑弯曲的右侧边。
For the OR gate, draw a concave left edge and a pointed right edge.
绘制或门时，要画内凹的左侧边和尖形的右侧边。
For NAND and NOR, add a small bubble on the output line after the main shape.
对于与非门和或非门，在主图形后方的输出线上添加一个小气泡。
For XOR, add an extra curved line on the input side, between the two input wires.
对于异或门，在输入侧两条输入线之间添加一条额外的弧线。
Label every input and output with a letter such as A, B and X.
用A、B、X等字母标记每个输入和输出。
10. Common Mistakes in Exams | 考试中的常见错误
Many students lose marks because of small symbol errors. Use this checklist to avoid the most frequent traps.
许多学生因为细小的符号错误而失分。使用这个检查清单来避免最常见的陷阱。
Confusing the AND-graph shape with the OR-graph shape: AND is a D shape, OR looks like a shield.
混淆与门和或门的形状：与门是D形，或门看起来像盾形。
Forgetting the bubble on NAND and NOR symbols. The bubble is essential.
忘记在与非门和或非门符号上画气泡。气泡是必不可少的。
Drawing XOR without the extra input-side curve, so it looks like a plain OR gate.
画异或门时漏掉输入侧的多余弧线，使它看起来像普通或门。
Writing the truth table in the wrong row order. Always use the order 00, 01, 10, 11.
真值表行顺序写错。始终按00、01、10、11的顺序排列。
Using non-standard symbols. CIE expects the distinctive shapes described above, not alternative rectangular blocks.
使用非标准符号。CIE期望的是上述独特形状，而不是其他矩形模块表示法。
Mastering these symbols will help you solve all logic-gate questions quickly and accurately in the IGCSE Computer Science exam.
掌握这些符号将帮助你在IGCSE计算机科学考试中快速、准确地解决所有逻辑门问题。
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