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A Closer Look at Schematic Diagram Symbols to Go Back to the Basics

Schematic diagram symbols on crumpled circuit board schematic


When I learned to fly I got tested on how well I could read an aeronautical chart, otherwise known as a pilot’s map. Those charts have all kinds of different symbols on them, and you had to have them all memorized in order to pass the test. The guy testing me went through several different symbols which I knew, and then pointed to a light blue circle and asked me to identify it. “Airport with facilities,” “airport with lights,” I just couldn’t remember what that aggravating little blue circle was for. Then he began to laugh as he pulled his hand off the chart and I realized that he had been covering up the word “Olympia” and that little blue circle was actually the letter “O.”

This guy had a little fun at my expense, but it isn’t so fun when you don’t understand the symbols that you are working with—especially on an electronic schematic. Fortunately, the basic symbols of resistors, capacitors, and other components are pretty easy to remember and you can find them listed in many different places. The real concern comes with how to work with these symbols in your schematic in order to provide the most useful information. We’ll take a look at some of the basics of schematic diagram symbols here and how you can best utilize them in your designs.

Symbols, Shapes, or Components

Symbols are drawn to be pretty simple while conveying the information of what they are and what they do. For instance, a capacitor symbol is two vertical lines side-by-side with horizontal lines going off the sides for the pins. A resistor is a set of nine diagonal lines connected together with horizontal lines going off the sides for pins. Logic gates are represented by either a rectangle with a rounded end, or a triangle, and will have multiple lines going off the sides for pins. Logic gates with pins that have a small bubble on the output pin lines indicate that its function is negated.

There are many, many schematic symbols and there are also many different ways that they are represented depending on the style and needs of the user. One company may encourage you to rotate a symbol on a schematic sheet while another company may not. The key is to learn what is the standard practice at your company and to stick with that.

Another important part of understanding the basics of schematic diagram symbols is to know the difference between symbols, shapes, and components. A symbol is the logical representation of an electronic component on the schematic sheet. Although shape is sometimes used to reference the appearance of a symbol, it is most often used to describe the physical shape of an electronic component in the PCB layout. A component is the physical part used on the circuit board. However, it is also common practice for both schematic users and PCB layout engineers to refer to their symbols and shapes as components. Yes, this can be a bit confusing if you aren’t used to it, but after some repetition, it will begin to make sense. As they say: context is everything.


Schematic symbols accountable for accurate documentation information

Learning how to optimize your schematic documentation can go a long way in your circuit board optimization


How Your PCB Design Tools Can Help

The schematic capture tools in your PCB design system typically have advance drafting features to help you create the schematic symbols that you need. You will have the ability to draw straight lines for capacitors, diagonal lines for resistors, curved lines for logic gates, and many other primitive drawing shapes as well.

Usually, the placement of pins in a schematic symbol is an automated function of the tools so that the pins will carry intelligent information with them. This information is used by the PCB layout tools to identify the pin, as well as by the simulation tools. Schematic symbols can also usually be loaded with other intelligent information such as a unique reference designator, footprint, a part number, tolerances, values, and whatever other information is required for their function. Complete information is stored in this symbol and transferred to layout in a netlist file.

As an added help, many PCB design tools include symbol building wizards or generators as a part of their software. These tools can be very useful to help speed you through the symbol creation process, and they also have the advantage of adding different attributes and data to the symbol that might be easy to neglect. Another helpful part of PCB design systems is the specific tools that will help you to collect and maintain these symbols in libraries as well as associating them to the physical footprint shapes that will be used in PCB layout.


Schematic diagram symbols identify parts for MCAD/ECAD engineers

The further your designs go, the more you’ll need strong output documentation


Other Methods of Creating Schematic Diagram Symbols

In addition to creating your own symbols, there are also other resources available to you for creating and collecting schematic diagram symbols. There are services that will create library parts for you based on your input. You can also invest in third party tools that will generate symbols and footprint shapes. There are also online services that provide access to existing library parts designed specifically for the tool system that you are using. Some of these services are available through the particular tool system that you are using, and others require a subscription fee to use.

No matter if you create your own schematic diagram symbols or you purchase them for online services, you will need these symbols in the creation of your electronic schematics. If you don’t already have a documented system of CAD library part standards in your company, it will serve you well to create one so that everyone can have access to. This will allow the uniform creation of all schematic symbols and PCB footprint shapes which can save you a lot of trouble trying to fit together differently created symbols and shapes later on.

Another thing that you will find to be very helpful is to use PCB design tools that have been created to help simplify your job as a designer. OrCAD is a schematic capture system that has years of experience behind it making it one of the premier PCB design systems in use today. You will find that OrCAD has all the drafting and symbol creation features that you need to easily create your library parts. Additionally, its connection with the library services of Ultra Librarian will quickly provide you with professionally made symbols so that you can move uninterrupted on to the task of creating your schematic circuitry.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.