science

NOR gate

NOR gate is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand NOR gate rather than just read about it. In short: The NOR (NOT OR) gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0) results.

Key takeaways

NOR gate belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NOR gate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NOR gate from memory before moving on to harder problems.

Reference excerpt

The NOR (NOT OR) gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator. It can also in some senses be seen as the inverse of an AND gate. NOR is a functionally complete operation—NOR gates can be combined to generate any other logical function. It shares this property with the NAND gate. By contrast, the OR operator is monotonic as it can only change LOW to HIGH but not vice versa. In most, but not all, circuit implementations, the negation comes for free—including CMOS and TTL. In such logic families, OR is the more complicated operation; it may use a NOR followed by a NOT. A significant exception is some forms of the domino logic family.

Symbols There are three symbols for NOR gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol. For more information see logic gate symbols. The ANSI symbol for the NOR gate is a standard OR gate with an inversion bubble connected. The bubble indicates that the function of the or gate has been inverted.

Hardware description and pinout NOR Gates are basic logic gates, and as such they are recognised in TTL and CMOS ICs. The standard, 4000 series, CMOS IC is the 4001, which includes four independent, two-input, NOR gates. The pinout diagram is as follows:

Availability These devices are available from most semiconductor manufacturers such as Fairchild Semiconductor, Philips or Texas Instruments. These are usually available in both through-hole DIP and SOIC format. Datasheets are readily available in most datasheet databases. In the popular CMOS and TTL logic families, NOR gates with up to 8 inputs are available:

CMOS 4001: quad 2-input NOR gate 4025: triple 3-input NOR gate 4002: dual 4-input NOR gate 4078: single 8-input NOR gate TTL 7402: quad 2-input NOR gate 7427: triple 3-input NOR gate 7425: dual 4-input NOR gate (with strobe, obsolete) 74260: dual 5-Input NOR gate 744078: single 8-input NOR gate In the older RTL and ECL families, NOR gates were efficient and most commonly used.

Implementations

The left diagram above show the construction of a 2-input NOR gate using NMOS logic circuitry. If either of the inputs is high, the corresponding N-channel MOSFET is turned on and the output is pulled low; otherwise the output is pulled high through the pull-up resistor. In the CMOS implementation on the right, the function of the pull-up resistor is implemented by the two p-type transistors in series on the top.

In CMOS, NOR gates are less efficient than NAND gates. This is due to the faster charge mobility in n-MOSFETs compared to p-MOSFETs, so that the parallel connection of two p-MOSFETs realised in the NAND gate is more favourable than their series connection in the NOR gate. For this reason, NAND gates are generally preferred over NOR gates in CMOS circuits.

Functional completeness

The NOR gate has the property of functional completeness, which it shares with the NAND gate. That is, any other logic function (AND, OR, etc.) can be implemented using only NOR gates. An entire processor can be created using NOR gates alone. The original Apollo Guidance Computer used 4,100 integrated circuits (IC), each one containing only two 3-input NOR gates. As NAND gates are also functionally complete, if no specific NOR gates are available, one can be made from NAND gates using NAND logic.

See also AND gate Boolean algebra (logic) Flash memory NAND gate NAND logic NOR logic NOT gate OR gate XNOR gate XOR gate

References

Illustrations

Worked examples

Example 1 — a first encounter with NOR gate

Start with the simplest possible case. Write down what NOR gate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NOR gate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NOR gate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NOR gate

In research: NOR gate appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NOR gate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NOR gate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Logic gates, so understanding it makes those chapters shorter.
In everyday life: Look for NOR gate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study NOR gate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NOR gate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NOR gate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NOR gate in simple terms?

Why does NOR gate matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NOR gate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NOR gate.