Introduction

The Celtic Engineer is a weekly newsletter produced by Celtic Engineering Solutions.  We hope you enjoy it.  If you have any suggestions for topics, would like to give feedback or want your email added to the distribution list please send an email to TCE@celticengineeringsolutions.com.

Size matters

 Don’t you love schematics that fit on a single page?

“I have a map of the United States… Actual size. It says, ‘Scale: 1 mile = 1 mile.’ I spent last summer folding it. I hardly ever unroll it. People ask me where I live, and I say, ‘E6.”

― Steven Wright

There is something very satisfying about a drawing that has everything in one place.  It is like an electronic device that has two pins, very little confusion. Start adding lots of pins and now you need a data sheet to figure out all the characters in the play.

Unfortunately for every Electrical Engineer, the day comes when it just won’t fit on a single sheet.  Breaking up a large design into logical pieces is very important. 

How did that get in here?

Groupings should make sense.  If you have two schematics and on one schematic you have all the analog input and on a second you have all the analog outputs, then having one input on the output page is confusing.  You want everything on a single page to have some commonality, some connection logically if not electrically.

That is not to say you can never do something that violates that rule of thumb.  There will be occasions where a chunk of schematic should and should not go somewhere.  (Have you ever wondered what the schematic for the apparatus that kills or does not kill a certain cat looks like?)  My advice is that when you feel you must include a part of a schematic in a place where it might not make sense, you should also include a note on the schematic that explains that point.  After all you are trying to convey information, not keep secrets.

<– He went that-a-way –>

When you are tracing out a signal on a schematic using your favorite CAD software, it is never a problem to figure out where a signal goes when it jumps from one page to another, or even from one side of a page to the other side of the same page.  The software will search for you.  But when you have a paper schematic it can be hard sometimes to know where to go when you have an off-page connector or just a net label.

One thing to do is to offer no help at all and leave the user to their own devices, but that is a bit harsh. Notes can help, page 5 E6 for example.  If all the connections on a page, go back to one other page, for example if you are working on the analog input and all of your net reverence go to the MCU page, then you are probably safe not saying anything, but if one signal goes off to somewhere else, consider putting a note telling the reader where to look.

Organization: Who’s on first

I tend to put the MCU on the first page. That is because it is the control hub of the device and to me the most important to understand how everything works.  I also tend to put the power regulators last. When I was a custodian, many moons ago, my supervisor said we have a thankless job.  No one will ever know you are doing your job correctly, until you don’t. That is when they notice it.  Regulators are the same way, while really important, if they are working correctly, no one will ever look at them.

The right way and the wrong way

I have seen other ways of organizing schematics.  One had a revision page on page one. Another had a table of contents on page one.  Both good ideas.  I have used both and like both.  Try them out and see what works for you and for your customers (those are the users, seek their feedback).  Try to be consistent. Having either a mental or a written set of rules for yourself can be useful.  Remember there is no right way and wrong way, unless your customers or your boss tells you otherwise, then there is.  What you want is to have someone who regularly uses your drawings to know what to expect and to be able to use them effectively and efficiently.

Ranch vs. Townhouse

I remember when we first started playing with Hierarchical schematics.  It was the best thing since sliced bread.  I wanted to use it everywhere.  I worked for a company dominated by mechanical engineers, not that there is anything wrong with being an ME.  We had a design review and I almost had to walk the plank. It was too complicated for them to easily follow. It was also unnecessary, for that particular design.

I refused to use Hierarchical designs for many years after that. Both were extreme positions.  Now I use flat designs most of the time, because they are easy for most people to understand.  I use Hierarchical design when the design calls for it.  If there is a circuit repeated many times, or a very complicated circuit repeated just a few times, it is often a better choice.  But be prepared to defined your decision if you have none EE’s consuming your drawings.

Final thoughts

At Celtic Engineering Solutions we have seen a lot of schematics.  If you need help laying out a schematic, or would like us to sit in on a design review give us a call or drop us an email.  We are easy to work with and we want to help you succeed.

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