engineering

Reference designator

Reference designator is a engineering 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 Reference designator rather than just read about it. In short: A reference designator (RefDes) unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board. The reference designator usually consists of one or two letters followed by a number, e.g.

Key takeaways

Reference designator belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Reference designator to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Reference designator from memory before moving on to harder problems.

Reference excerpt

A reference designator (RefDes) unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board. The reference designator usually consists of one or two letters followed by a number, e.g. C3, D1, R4, U15. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, e.g. R17A, R17B. The IEEE 315 standard contains a list of Class Designation Letters to use for electrical and electronic assemblies. For example, the letter R is a reference prefix for the resistors of an assembly, C for capacitors, K for relays. Industrial electrical installations often use reference designators according to IEC 81346 and NFPA 79 Annex E: Device and Component Designations.

History IEEE 200-1975 or "Standard Reference Designations for Electrical and Electronics Parts and Equipments" is a standard that was used to define referencing naming systems for collections of electronic equipment. IEEE 200 was ratified in 1975. The IEEE renewed the standard in the 1990s, but withdrew it from active support shortly thereafter. This document also has an ANSI document number, ANSI Y32.16-1975. This standard codified information from, among other sources, a United States military standard MIL-STD-16 which dates back to at least the 1950s in American industry. To replace IEEE 200-1975, ASME, a standards body for mechanical engineers, initiated the new standard ASME Y14.44-2008. This standard, along with IEEE 315-1975, provide the electrical designer with guidance on how to properly reference and annotate everything from a single circuit board to a collection of complete enclosures.

Definition ASME Y14.44-2008 and IEEE 315-1975 define how to reference and annotate components of electronic devices. It breaks down a system into units, and then any number of sub-assemblies. The unit is the highest level of demarcation in a system and is always a numeral. Subsequent demarcation are called assemblies and always have the Class Letter "A" as a prefix following by a sequential number starting with 1. Any number of sub-assemblies may be defined until finally reaching the component. Note that IEEE 315-1975 defines separate class designation letters for separable assemblies (class designation 'A') and inseparable assemblies (class designation 'U'). Inseparable assemblies—i.e., "items which are ordinarily replaced as a single item of supply"—are typically treated as components in this referencing scheme. Examples:

1A12A2R3 - Unit 1, Assembly 12, Sub-assembly 2, Resistor 3 1A12A2U3 - Unit 1, Assembly 12, Sub-assembly 2, Inseparable Assembly 3 Especially valuable is the method of referencing and annotating cables plus their connectors within and outside assemblies. Examples:

1A1A44J5 - Unit 1, Assembly 1, Sub-Assembly 44, Jack 5 (J5 is a connector on a box referenced as A44) 1A1A45J333 - Unit 1, Assembly 1, Sub-Assembly 45, Jack 333 (J333 is a connector on a box referenced as A45) A cable connecting these two might be:

1A1W35 - In the assembly A1 is a cable called W35. Connectors on this cable would be designated:

1A1W35P1 1A1W35P2 ASME Y14.44-2008 continues the convention of Plug P and Jack J when assigning references for electrical connectors in assemblies where a J (or jack) is the more fixed and P (or plug) is the less fixed of a connector pair, without regard to the gender of the connector contacts. The construction of reference designators is covered by IEEE 200-1975/ANSI Y32.16-1975 (replaced by ASME Y14.44-2008) and IEEE 315-1975.

Designators The table below lists designators commonly used, and does not necessarily comply with standards. For modern use, designators are often simplified towards shorter designators, because it requires less space on silkscreens.

Other designators

Illustrations

Worked examples

Example 1 — a first encounter with Reference designator

Start with the simplest possible case. Write down what Reference designator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Reference designator 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 Reference designator 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 Reference designator

In research: Reference designator appears in engineering 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 Reference designator 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: Reference designator is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Reference designator 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.

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How to study Reference designator in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Reference designator 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 Reference designator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Reference designator in simple terms?

Why does Reference designator matter?

Because it connects several engineering 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 Reference designator?

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 Reference designator.