Reference Designators

This is not a subject I give much thought to.

For one, we here at Screaming Circuits don’t really care too much what convention you use for your components. We want them to match and be properly formatted when in your BoM, of course. But because we program our SMT machines electronically, we don’t really care if you mix things up; i.e., O instead of R for resistor, or F for capacitor. It’s not a good idea to do that, but we can still build it.

But, if we can build it, shouldn’t anybody be able to build it? And, if anyone can build it, why should it matter? Well, in theory, it shouldn’t matter at all. In practice though, people tend to be human and humans tend to be error-prone. That’s why we have standards, conventions and test procedures — to reduce the chances for errors. We also have conventions for the purpose of distributing bad, overpriced food and educational sessions, but that’s probably a different convention.

It would be kind of like if you drove into a small town and there was a sign at the city limits indicating that in this town, red means go and green means stop. You would have all of the information needed safely traverse the town, but you would still be very prone to go with the green light.

I just recently saw a design where the connectors were labeled U1, U2… Again, we can build this and we did. But, if it comes time to do any rework, or if you want to make some design mods in-house, of if someone else needs to work with the board, they’ll see “U something” and think you’re talking about an IC instead of some sort of connector.

There are some specific industry standard documents covering the reference designator conventions, but I bet it’s one of those things that most people just sort of know, but don’t have the official document to go with it. Wikipedia has a list and a lot of companies probably have their own conventions.

It is easy enough to find these lists of conventions, but it does leave me wondering how some of them came to be. I get “R” for resistor and “C” for capacitor. “Q” for transistor even makes sense, although it’s derived from a property of the device instead of the name as are R and C. But, why “U” for integrated circuit? It used to be “IC”, but that’s fallen out of favor now. Really weird is the inductor. It starts with “I”, it’s inductance value is measured in “henries” and henries are indicated by “L.” Go figure.

Duane Benson
U take the high road and L take the low road

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About Mike

Mike Buetow is president of the Printed Circuit Engineering Association ( He previously was editor-in-chief of Circuits Assembly magazine, the leading publication for electronics manufacturing, and PCD&F, the leading publication for printed circuit design and fabrication. He spent 21 years as vice president and editorial director of UP Media Group, for which he oversaw all editorial and production aspects. He has more than 30 years' experience in the electronics industry, including six years at IPC, an electronics trade association, at which he was a technical projects manager and communications director. He has also held editorial positions at SMT Magazine, community newspapers and in book publishing. He is a graduate of the University of Illinois. Follow Mike on Twitter: @mikebuetow

3 thoughts on “Reference Designators

  1. I think Mike may be correct about the U.

    Connectors are always confusing, especially when people use the same references as used for other components or the same for plug and socket.

    In general many try to keep it to a single letter – that at least gives us 26 different types of component to place on the board.
    The smaller the reference – the smaller the area it takes up. Only getting longer when space is available such as with relays (RLn), transformers (TX) etc.

  2. A few additional thoughts on reference designators.

    Some letter prefixes are not suitable for reference designators, as they can create confusion. For example, the letters I, O, can be confused with the numbers one or zero.

    Connectors are a world unto themselves. While it can be helpful to identify connectors by type – i.e., Pxx for Plug or Jxx for Jack – not all connectors fit into this category.

    When assigning reference designators, sometimes it is important to consider the system or product interconnects as a whole. For example, a multi-board system should not include a ‘J1′ on multiple PCB’s, because this can lead to confusion during system assembly or debugging.

    Another issue to consider is assembly/test/end user documentation. Are there documents which refer to connectors or other key components by refdes? If so, it’s a good idea to keep those component references unchanged.

    Regardless of the reference scheme used, it is always best to settle the reference designators early, and use back-annotation (if at all) in a well-defined manner. Changing refdes’ in mid-stream can create all sorts of problems.

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