How to Read a Schematic

Free your mind from thinking that what you wire has to look like the schematic.

Electrons are wonderful things. For the most part, they don't care too much about what kind of wire they are going through or if it's a piece of sheet metal, a car body or engine, a screw, spade lug, solder, circuit board, or even track. The wire doesn't necessarily need to be straight. It can have a serious kink or make a left turn. So the first major hurdle you need to overcome is to free your mind from thinking that what you wire has to look like the schematic.

Let us stick with this for a moment and make sure you have this important concept. I see that it is a stumbling block for many of you. Unless specified, a schematic...

-does not show physical arrangement.

-does not show the physical size of electrical devices that appear on the schematic.

-does not show wire size.

-does not show how things will actually be interconnected.

-does not show what is next to what.

-does not show order of devices connected to a particular wire.

-does not show where interconnections are to be made.

-does not show how many connections will actually be made.

That's a basic schematic. So unless specified, you don't have to make your work look like the schematic.

The goal of the person drawing the schematic is to make it as simple and easy to read as possible. At least, that's what it should be. Other things often get in the way like trying to fit my examples into a screenfull in this web page.

Of course, many of the above listed things are important. If they are, there will be additional information that will either be on the schematic or in accompanying text.

And yes, sometimes you may need ESP and infer a few things. Unfortunately, there are some schematics that leave so much to the imagination that they could be considered craftsman kits! Just like the craftsman kits, it would have been easier if the information had been provided — and it should have been. Hopefully by making you a little smarter here today, you can resolve some of these situations.

When is a wire on a schematic connected to another wire?

This is another big trouble spot for many of you. Let's see if we can take care of that.

Prior to computers, there were two general conventions for showing when wires were or were not connected together.

The age of computers essentially did away with the one with half circles. The reasons are obvious. For starters, it is ugly. Second, it is much more time consuming to draw on most CAD (Computer Aided Design) packages.

2 intersecting lines, no dot = not connected

2 intersecting lines, dot = connected

Both of these are connected.  I prefer to put the dot.  Common sense says that even the one without the dot is connected.  What purpose would be served by the vertical line if it wasn't connected???  The omission of the dot would be a simple mistake.

Does this mean two wires are joined or is it a single wire? If the horizontal wire is a bus wire, then yes, it probably is two wires joined. Otherwise, it may simply be a single wire that was drawn with kink in it — perhaps for no other particular reason than it was convenient for the drawer..

I know some people are "attached" to the half circles. Well, it's time to say goodbye to them. They are nothing more than training wheels.

Bussing

You might think that buses are a demon of the computer age. But any layout that had more than one set of feeders going to the track had a bus. It's just that no one called it a bus. You just said "connect the track feeders to the track terminals on the power pack."

If your layout had block control, the pair of wires that were "daisy chained" from switch to switch in your control panel, that was your bus. Don't ask where the term "daisy chained" came from. To me, a chain of daisies might be a lei.

There is a subtle difference between a daisy chain and a bus. Conceptually, a bus is an unbroken wire that has taps on it.

Daisy chain means that pieces of wire interconnect two neighboring things. A connects to B, B then is connected to C, and then C is connected to D, etc.

More to come — someday.  There is a never ending supply of material I could be putting into the main page.  Sorry I haven't continued this section.  I just haven't had the time.