Design Con 2015

Why I Hate Your Schematic Diagrams

-October 13, 2011

Actually, I like schematic diagrams, but today's schematic "capture" software makes schematic diagrams worse than those created by because it gives designers too much flexibility and too many opportunities for carelessness.  Here's my list of what makes schematic diagrams so easy to hate:

1.  Few multipage schematics include an overall flow diagram that shows data and control-signal paths and relates those major sections to subsequent pages. I guess designers figure no one needs to get a grasp of the overall system.

2.  The first microprocessor manual I perused in the early '70's includes a 2-page reference schematic for a large board with dozens of ICs. The diagram included every component and signal name. Today, designers think they should create a page for resistors, a page for capacitors, a page for connectors, and so on.  Probably easy to draw, but almost impossible to follow.  How about putting more things that connect to one another on one page?  I just saw a schematic that includes a separate section with three resistors, three resistors and three signal names.  The diagram offers no clue what those signals connect to or what the LED labeled "GREEN LED" indicates. How about "POWER" or "FAULT?"

3.  Thanks for wasting space with component designations such as 100n-0603 X5R 16V and LTC2942CDCB-1#TRMPBF-DFN6. I don't plan to duplicate your circuit, so a label such as 100 nF/16V or LTC2942 would do the job and reduce the clutter. On a schematic diagram I have in hand, three passive components take almost a cubic inch. The symbols seem too large and their labels take up too much space.

4.  Schematic-capture programs often cram component names so close to component blocks that they're difficult to read. Is that a "T" or a "1"?  Maybe it's a "7."  Who knows.

5.  A variety of colors might help you draw a schematic, but not everyone has a color printer and even those who do might have a difficult time seeing the components for the rainbow of component, connection, signal, and ground colors. Stick with black and white diagrams.

6.  You forgot to include a diagram of your board layouts that indicate component and signal locations.  Your silk-screen legends often get hidden by components, which makes it difficult to locate a test point or find a tiny connection. Lay it all out on paper.

7.  Give signals cryptic names such as "gauge_cc_al_n" and don't include any information about where to find other "gauge_cc_al_n" signals. This type of nomenclature, shown in red on many schematics, doesn't print well, gives no clue about the type or use of the signal, and is that an, a1, aI, ai, or something else?  Mixing lots of 0's, O's, o's, I's, i's, L's, and 1's on a schematic doesn't help either.

8.  When you pass schematic diagrams to your documentation or production people, ensure they run off the margins of a page, or use a non-standard paper size. I have a schematic diagram that runs over the left and right sides of an 8.5 x 11-inch page and leaves 17 square inches unused at the top. Thankfully only a few ground connections went missing, but why should I have to go back and pencil them in by hand?

9.  Don't include a list of signal names and any cross reference to the what the signals do, what they connect to, or what sheets to find them on.  We enjoy trying to find an oddly-named signal in a 10-page schematic at 3:00 PM on Friday afternoon. Engineers must love test points labeled TP1, TP2, and so on.  Folks, how about a signal NAME?

10.  Use four-signal connections.  On small diagrams, it becomes difficult to distinguish a fly-over from one of these 4-way contacts.  If you connect only three lines at a point, you avoid ambiguity. --Jon Titus

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