📚 Standard Logic Gate Symbols Recognition & Memory | 标准逻辑门符号识别与记忆
Logic gates are the fundamental building blocks of all digital circuits, from simple calculators to advanced microprocessors. In CIE Computer Science, you must be able to recognise each standard gate symbol instantly and recall its truth table, Boolean expression, and behaviour without hesitation. This guide provides clear symbol diagrams, memory tricks, and exam-focused comparisons to help you master logic gate recognition efficiently.
逻辑门是所有数字电路的基础构件，从简单的计算器到高级微处理器都不例外。在 CIE 计算机科学课程中，你必须能够瞬间识别每个标准逻辑门符号，并毫不犹豫地回忆其真值表、布尔表达式和行为。本指南提供清晰的符号图示、记忆技巧和紧扣考点的对比，帮助你高效掌握逻辑门识别。
1. What Are Logic Gates? | 什么是逻辑门？
A logic gate is an electronic circuit that takes one or more binary inputs (0 or 1) and produces a single binary output based on a fixed logical rule. The relationship between inputs and output is described using a truth table, a Boolean expression, and a standard graphical symbol.
逻辑门是一种电子电路，它接收一个或多个二进制输入（0 或 1），并依据固定的逻辑规则产生单个二进制输出。输入与输出之间的关系通过真值表、布尔表达式和标准图形符号来描述。
Binary inputs are usually labelled A, B, C... and the output is labelled X.
二进制输入通常标记为 A、B、C…，输出标记为 X。
There are only two possible values: 0 (low / false) and 1 (high / true).
只有两种可能的取值：0（低电平/假）和 1（高电平/真）。
In CIE exams, you will be tested on: NOT, AND, OR, NAND, NOR, XOR, and sometimes XNOR.
在 CIE 考试中，你会被考查：NOT、AND、OR、NAND、NOR、XOR，有时还有 XNOR。
2. Gate Inputs and Outputs | 门的输入与输出
Every logic gate follows a simple pattern: inputs enter from the left side of the symbol, and the output leaves from the right side. Recognising this convention helps you read unfamiliar gate diagrams correctly.
每个逻辑门都遵循一个简单的模式：输入从符号的左侧进入，输出从右侧离开。识别这一约定有助于你正确阅读不熟悉的门电路图。
Output = f(Input A, Input B, ...)
For a 2-input gate, there are four possible input combinations: 00, 01, 10, 11. For a 3-input gate, there are eight combinations. The truth table lists every combination and the corresponding output.
对于双输入门，共有四种可能的输入组合：00、01、10、11。对于三输入门，共有八种组合。真值表列出每一种组合及对应的输出。
3. NOT Gate (Inverter) | NOT 门（反相器）
The NOT gate is the simplest gate. It has one input and one output. The output is always the opposite of the input. Its symbol is a triangle pointing right, followed by a small circle (bubble) at the tip. The bubble indicates inversion.
NOT 门是最简单的门。它有一个输入和一个输出，输出始终与输入相反。它的符号是一个向右的三角形，尖端带有一个小圆圈（气泡）。小圆圈表示取反。
| A | X = NOT A |
| 0 | 1 |
| 1 | 0 |
Memory trick: "NOT" means "opposite" — the triangle tells you it is a gate, and the bubble says "flip the value".
记忆技巧：“NOT”意为“取反”——三角形表示这是一个门，小圆圈表示“翻转数值”。
Boolean expression: X = Ā or X = NOT A.
布尔表达式：X = Ā 或 X = NOT A。
Exam tip: Never confuse the NOT gate with a buffer. A buffer has no bubble and simply passes the input unchanged; CIE rarely asks about buffers.
考试提示：切勿将 NOT 门与缓冲器混淆。缓冲器没有小圆圈，只是原样传递输入；CIE 很少考查缓冲器。
4. AND Gate | AND 门
The AND gate produces an output of 1 only when all of its inputs are 1. Its symbol is a flat-left D shape: a straight vertical input side and a curved output side. Think of it like the letter "D".
AND 门仅在所有输入都为 1 时才输出 1。它的符号是左侧平直的 D 形：左侧竖直边为输入边，右侧弧形为输出边。可以把它想象成字母“D”。
| A | B | X = A AND B |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Memory trick: "AND" requires every input to be true — the D shape looks like a gate that only opens when all keys are inserted.
记忆技巧：“AND”要求每个输入都为真——D 形看起来像一扇只有插入所有钥匙才会打开的门。
Boolean expression: X = A · B (often written as AB).
布尔表达式：X = A · B（常写作 AB）。
The output is 1 only in the last row of the truth table, when A = 1 AND B = 1.
只有在真值表最后一行，即 A = 1 且 B = 1 时，输出才为 1。
5. OR Gate | OR 门
The OR gate produces an output of 1 when at least one input is 1. Its symbol is a curved shield-like shape: the left side is concave and the right side is pointed. Think of it as a "shield" or "arrowhead".
OR 门在至少一个输入为 1 时输出 1。它的符号是弯曲的盾牌状：左侧内凹，右侧尖凸。可以把它想象成“盾牌”或“箭头”。
| A | B | X = A OR B |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
Memory trick: "OR" is generous — if ANY input is 1, the output is 1. The shield shape looks like an open mouth ready to accept anything.
记忆技巧：“OR”很慷慨——只要任意输入为 1，输出就是 1。盾牌形状看起来像一张张开的嘴，准备接受任何东西。
Boolean expression: X = A + B.
布尔表达式：X = A + B。
The only 0 output appears when both inputs are 0.
唯一输出 0 的情况是当两个输入都为 0 时。
6. NAND Gate | NAND 门
The NAND gate is an AND gate followed by a NOT gate. It produces an output of 0 only when both inputs are 1. Its symbol is exactly the AND gate (D shape) with a small bubble attached to the output side.
NAND 门是 AND 门后接一个 NOT 门。只有当两个输入都为 1 时，输出才为 0。它的符号与 AND 门完全相同（D 形），只是在输出侧多了一个小圆圈。
| A | B | X = A NAND B |
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Memory trick: "NAND" = "NOT AND". If AND output is 1, NAND flips it to 0. The bubble on the D shape is the memory cue.
记忆技巧：“NAND”=“NOT AND”。如果 AND 输出为 1，NAND 将其翻转为 0。D 形上的小圆圈就是记忆线索。
Boolean expression: X = A · B (with a bar over the whole expression).
布尔表达式：X = A · B（整个表达式上方带横线）。
Exam tip: NAND is called a "universal gate" because any other gate can be built using only NAND gates. CIE may ask you to recognise this fact.
考试提示：NAND 被称为“通用门”，因为仅使用 NAND 门就可以构建任何其他门。CIE 可能会考查这一点。
7. NOR Gate | NOR 门
The NOR gate is an OR gate followed by a NOT gate. It produces an output of 1 only when both inputs are 0. Its symbol is the OR gate (shield shape) with a small bubble on the output side.
NOR 门是 OR 门后接一个 NOT 门。只有当两个输入都为 0 时，输出才为 1。它的符号是 OR 门（盾牌形）在输出侧带一个小圆圈。
| A | B | X = A NOR B |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
Memory trick: "NOR" = "NOT OR". The output is 1 only when OR would give 0 — i.e., when every input is 0. Bubble on the shield = NOR.
记忆技巧：“NOR”=“NOT OR”。只有当 OR 会给出 0 时——即每个输入都为 0——输出才为 1。盾牌上的小圆圈 = NOR。
Boolean expression: X = A + B (with a bar over the whole expression).
布尔表达式：X = A + B（整个表达式上方带横线）。
NOR is also a universal gate, like NAND.
NOR 和 NAND 一样，也是通用门。
8. XOR Gate | XOR 门
The XOR (exclusive OR) gate produces an output of 1 when the inputs are different. Its symbol is like the OR gate (shield shape) but with an extra curved line on the input side, parallel to the left edge. This extra curve is the key distinguishing feature.
XOR（异或）门在输入不同时输出 1。它的符号与 OR 门类似（盾牌形），但在输入侧多了一条平行于左边缘的额外曲线。这条额外曲线是关键的区别特征。
| A | B | X = A XOR B |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Memory trick: XOR is "eXclusive OR" — it excludes the case where both are 1. The extra curve on the symbol is like an "exclusive fence".
记忆技巧：XOR 是“eXclusive OR”——它排除了两个输入都为 1 的情况。符号上的额外曲线就像一道“专属围栏”。
Boolean expression: X = A ⊕ B (sometimes written as A·B̄ + Ā·B).
布尔表达式：X = A ⊕ B（有时写作 A·B̄ + Ā·B）。
XOR outputs 1 when inputs are different, and 0 when inputs are the same. This makes it useful for binary addition.
XOR 在输入不同时输出 1，输入相同时输出 0。这使其可用于二进制加法。
9. XNOR Gate | XNOR 门
The XNOR gate is the inverse of XOR. It produces an output of 1 when the inputs are the same. Its symbol is the XOR shield shape with a bubble on the output side. It is sometimes called the "equivalence" gate.
XNOR 门是 XOR 的取反。它在输入相同时输出 1。它的符号是 XOR 盾牌形在输出侧带一个小圆圈。它有时被称为“同或”门。
| A | B | X = A XNOR B |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Memory trick: XNOR is "XOR with a bubble". When inputs are equal, output is 1. Think "equal → one".
记忆技巧：XNOR 是“带小圆圈的 XOR”。当输入相等时，输出为 1。想想“相同 → 1”。
Boolean expression: X = A ⊙ B or simply the complement of XOR.
布尔表达式：X = A ⊙ B，或简单地说就是 XOR 的补。
XNOR is less common in CIE than the other six gates, but you should recognise it in diagrams.
XNOR 在 CIE 考试中不如其他六种门常见，但你应当能在图形中识别它。
10. Complete Symbol Comparison Table | 符号对比总表
This table summarises the distinctive shape features of each gate. Use it as a quick reference for revision.
下表总结了每个门独特的形状特征，可作为复习时的快速参考。
| Gate | Key Shape Feature | 关键形状特征 |
| NOT | Triangle + bubble | 三角形 + 小圆圈 |
| AND | D shape, no bubble | D 形，无小圆圈 |
| OR | Shield shape, no bubble | 盾牌形，无小圆圈 |
| NAND | D shape + bubble | D 形 + 小圆圈 |
| NOR | Shield shape + bubble | 盾牌形 + 小圆圈 |
| XOR | Shield shape + extra curve, no bubble | 盾牌形 + 额外曲线，无小圆圈 |
| XNOR | Shield shape + extra curve + bubble | 盾牌形 + 额外曲线 + 小圆圈 |
A bubble always means "invert the output" of the base gate.
小圆圈总是表示“对基础门的输出取反”。
An extra curve on the left always means "exclusive" (XOR family).
左侧的额外曲线总是表示“异”（XOR 系列）。
If you know AND and OR shapes, you can derive NAND, NOR, XOR, and XNOR by adding bubbles and curves.
如果你知道 AND 和 OR 的形状，就可以通过添加小圆圈和曲线推导出 NAND、NOR、XOR 和 XNOR。
11. Memory Techniques and Common Confusions | 记忆技巧与常见混淆
Many students confuse AND with OR, or forget whether a bubble is placed on NAND or NOR. Here are practical memory strategies and exam traps to avoid.
许多学生会混淆 AND 与 OR，或者忘记小圆圈是加在 NAND 还是 NOR 上。以下是一些实用的记忆策略和需要避免的考试陷阱。
AND = "Ally": both inputs must agree to give 1. The D shape is flat like a closed door.
AND = “盟友”：两个输入必须一致才能给出 1。D 形平直，像一扇关着的门。
OR = "Open": any input opens the way to 1. The shield shape is curved like an open arch.
OR = “敞开”：任意输入都能通向 1。盾牌形弯曲，像一座敞开的拱门。
Bubble = "NOT": wherever you see a bubble, invert the result before writing the output.
小圆圈 = “NOT”：无论在哪里看到小圆圈，在写出输出前先对结果取反。
NAND is "Not AND" — it is the D shape with a bubble. NOR is "Not OR" — it is the shield with a bubble.
NAND 是“Not AND”——它是带小圆圈的 D 形。NOR 是“Not OR”——它是带小圆圈的盾牌形。
XOR has an extra curve; XNOR has an extra curve plus a bubble. If you see a curve, say "exclusive".
XOR 有额外曲线；XNOR 有额外曲线加小圆圈。如果看到曲线，就说“异”。
De Morgan's Law: NOT (A AND B) = (NOT A) OR (NOT B), and NOT (A OR B) = (NOT A) AND (NOT B)
De Morgan's law helps you understand why NAND and NOR symbols look the way they do. A NAND gate can also be drawn as an OR gate with bubbles on its inputs — both forms are equivalent. This is a very common exam question.
德摩根定律帮助你理解为什么 NAND 和 NOR 符号看起来如此。NAND 门也可以画成在输入端带小圆圈的 OR 门——两种形式是等价的。这是一个非常常见的考试问题。
12. Quick Revision Checklist | 快速复习清单
Use this checklist before your exam to ensure you can confidently handle any logic gate question.
考试前使用这份清单，确保你能够自信地处理任何逻辑门问题。
Can you draw each gate symbol from memory?
你能凭记忆画出每个门符号吗？
Can you write the truth table for each gate, including 3-input versions?
你能为每个门写出真值表，包括三输入版本吗？
Can you match a gate symbol to its Boolean expression?
你能将门符号与其布尔表达式匹配吗？
Can you identify gates when the bubble is placed on an input instead of the output (De Morgan equivalents)?
当小圆圈放在输入端而不是输出端时（德摩根等价形式），你能识别出对应的门吗？
Can you explain why NAND and NOR are universal gates?
你能解释为什么 NAND 和 NOR 是通用门吗？
Mastering logic gate symbols is mostly about visual pattern recognition. Focus on the base shapes (triangle, D, shield), then add the modifying features: bubble for NOT, extra curve for exclusive. In every truth table, compare the output column with the AND and OR outputs to confirm your answer.
掌握逻辑门符号主要是视觉模式识别。专注于基础形状（三角形、D 形、盾牌形），然后添加修饰特征：小圆圈表示 NOT，额外曲线表示异。在每个真值表中，将输出列与 AND 和 OR 的输出进行比较，以确认你的答案。
Published by TutorHao | Computer Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图