On July 23rd 1983, Air Canada flight 143 unexpectedly found itself out of fuel while cruising over Ontario. Although mechanical problems with fuel gauges contributed to the incident, the main problem stemmed from a mistake made on the ground during refueling. At that time, Canada’s aviation sector was converting from imperial to metric units, and there was some confusion with the change. As a result, the fuel required was incorrectly calculated with the conversion factor for pounds instead of liters, and flight 143 took off with only half the fuel load that they needed.
Trying to make the right call without the correct standards to work with can be a challenge, and in the case of this airliner, it was almost a disaster. Fortunately, flight 143 was able to make a successful emergency landing at a closed air force base that was being used as a dragstrip. The only afflictions suffered that day were from the people on the ground who were scared out of their wits when the giant Boeing 767 abruptly landed in front of them after silently gliding in.
When capturing a schematic, it is also possible to find yourself in a difficult situation if you use symbols that aren’t universally recognized. You may find that the non-standard symbols are causing confusion for PCB layout designers, test and debug engineers, and field service technicians. These symbols may also have a negative impact on the CAD tools as well. Let’s look a little deeper into the reason why schematic symbols should be built to a standard such as IEEE 315-1975, and how you can do this.
Problems with Using Non-Standard Schematic Symbols
Like handwriting, many people will have developed their own flair for how they create a schematic symbol. A creatively drawn number on a blackboard once caused me to miss a question on a test in high school because I misinterpreted the equation. In the same way, a symbol that is not drawn according to an accepted standard can also be misinterpreted by those reading the schematic, or by the CAD tools. Here are some problems that can arise from creating schematic symbols that are not in accordance with the standard:
Functionality: Non-standard notation on pins can cause the functionality of the part to be misinterpreted. Some examples of this would be the polarity of pins on a capacitor or swapping the cathode and anode of a diode.
Reference designator: If the wrong designation letter is used for a reference designator, it can cause confusion as to what the part really is. In some cases, this is due to the choice of the user, while in others it may be because of not configuring the symbol correctly in the CAD tools.
Values: If a symbol’s value, such as a tolerance or power rating, is incorrect or omitted, it can also lead to confusion. The wrong part may be installed on the board, or it may lead to unexpected results when testing the circuit.
Appearance: When the end users of the schematic are looking for a specific part, they may have difficulty in locating it if it isn’t drawn in an expected way.
In addition to problems with human interaction, there may be CAD system problems as well. For instance, a non-standard symbol may not have its pins placed on the schematic grid causing problems in the CAD system when trying to connect nets to those pins. Another problem is if the symbol is created correctly for the standard, but doesn’t follow the correct procedure in the CAD system. An example of this is when plain graphical objects are used for pins instead of intelligent pin objects.
The only thing worse than a non-standard symbol, is when those symbols are hand drawn making their interpretation even more difficult. On the other hand though, using a standard to help you create your CAD symbols can help you to avoid a lot of these problems.
A hand drawn schematic can cause confusion, especially if the symbols are non-standard
How a Standard like IEEE 315-1975 Can Help in the Creation of Symbols
Using standards as the template for your schematic symbols can prove to be a huge benefit when you are building your CAD libraries. IEEE 315-1975 has been in place for a long time and provides graphics as well as reference designation letters for electrical and electronic symbols. There are also newer standards, such as IEC 60617, which is an international standard for electronic symbols. Although there is a lot of movement towards the IEC standard, the IEEE standard is still widely used in many companies.
These standards show the graphical representation of a symbol, as well as reference designation letters. The IEC standard is a database of over 1900 illustrated graphical symbols and contains information such as names, status levels, and application notes. Being a database, the IEC standard can be searched making it a very user friendly tool to use and contains PDF images and other documentation that can be downloaded.
These standards will help to ensure that the symbols you build will be correct in how they represent the functionality of the device. You can also trust that they will be compatible with other schematics, and they will have correct reference designators, values, and appearances. And with correctly created symbols, you can create a cleaner and more effective schematic. But there is an easier way to populate your schematic symbol libraries now instead of researching and building the symbols yourself.
With an online search service, you can easily place a resistor straight from the online catalog
Alternatives to Symbol Creation Using Advanced Schematic Tools
More and more, schematic capture tools are now featuring online connections to PCB part library services. These services usually pull component information directly from the vendors and create the parts using the applicable standards for the CAD tools that the user is working with. This assures the PCB designer that the parts that they are using are not only validated by the component manufacturer but are also correctly built according to the latest industry standards.
Within the schematic tools that are equipped with these online features, the user will usually have an additional online browser option when placing symbols on their schematic. This browser enables them to search through online catalogs for the specific part they are looking for by filtering their search patterns according to their desired parameters. Once the part they need is found and selected, they can download it complete with a symbol, PCB footprint, 3D models, and datasheets.
Thankfully there are PCB design tools already available to you on the market today that have these online part browsing capabilities. One of the best of these PCB design systems is from Cadence. OrCAD PCB Designer has a Unified Parts Search system built into it allowing you to access online library services the way we described. In addition to the symbol and model data of the parts that you need, you can also get a lot of technical information about your desired component before you use it. This will give you a tremendous advantage by knowing whether or not a specific part will be a good fit for your product before you commit to it.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.