AmpHour#80 Schematics annotation

[FreeThinker]

In the Amphour Dave and Chris discussed Schematics and the amount of info held there in. This is a rant of mine (I work as a maintenance tech) as some are very easy to read some are a real pain. The main problem is that the Designer knows how his circuit works and lays out his schematic with scant regard to the person who is reading it trying to figure out what is going on.Some of the drawing I have to work with are 30 or forty or even more pages long and if the signal you are tracing disappears off the page you can have a hell of a time finding it if its not labelled correctly (or not at all). ALL signals should be location tagged even if they are on the same page, it just makes tracing SO much easier. As an example (not a criticism but it high lights the problem) take Daves b rev board for the power supply. In the ADC section is a signal ADC-VIN where does it come from?. Dave would say it's obvious its from the PSU and is clearly marked, but I don't know that off the top of my head and have to search across the drawing to find it interrupting my train of thought. And the answer so simple. Just tag each signal with its location so an input would show its source and an output would show it's destination. That is the purpose of the 1,2,3 across the top and the A,B,C down the side but in so many cases they are just not used and it really is just sloppy and bloody annoying. <Rant Over>

[Rufus]

take Daves b rev board for the power supply. In the ADC section is a signal ADC-VIN where does it come from?

I hate to see net labels stuck on apparently complete nets indicating that there might or might not be a connection somewhere else. They should be placed on wire stubs indicating there definitely is a connection somewhere else. I try to group and align such labels on stubs so it is easy to scan the labels looking for the one you want.  Oh and boy, do I hate that 8 pin DIL symbol for the current sense amp.

Drawing good schematics is an art. As an engineer I draw schematics to present the design to other engineers (why would I want an NC pin on a part shown on the schematic for example). CAD people tend to look at schematics as a graphical system for entering net and parts lists the most important consideration being producing a correct PCB from it. For maintenance and test being able to easily find and trace things is more important.

I sometimes think you need three versions of any schematic.

[FreeThinker]

take Daves b rev board for the power supply. In the ADC section is a signal ADC-VIN where does it come from?

I hate to see net labels stuck on apparently complete nets indicating that there might or might not be a connection somewhere else. They should be placed on wire stubs indicating there definitely is a connection somewhere else.

And you need to say where that somewhere else IS. Searching a schematic for a net name gets to be a PITA very quickly. Simply prefixing a label  <6c next to the net name at 3A makes it so much more readable. The < indicates it's an output and 6c the destination at 6c it would be labelled >3A indicating it's an input from that area...Simples.

[HLA-27b]

Using shorter lines with least number of kinks tends to produce better schematics. Thin long wires with lots of kinks winding around the page are difficult to follow and make circuit topology hard to grasp. When unavoidable tags may be used but they receive almost as much criticism as spaghetti layouts and rightly so.

Good use of bus lines can make or break a schematic. Good usage is an art form  and bad usage is worse than not using them at all.

Scale and "weight" of the circuit on the page is important. Cramming something into every corner of the page is not good, very sparse is not good either. Page utilization of 50% to 70% seems to be the sweet spot.

All of these are of course graphic design issues and should be handled separately from the main project because they require a different mindset and time, my 2¢.

[Alex]

Many times I will modify the standard schematic symbol of a component (primarily ICs) so that the pins are facing the same direction as whatever they are connected to. This helps in keeping the schematic clean by minimising wires that wrap around components.

Some of the steps you would take to produce clean software code also apply to schematics and PCBs. If you hack everything together without a master plan (e.g. block diagram, flow charts, PCB areas) then it will get messy and confusing very quickly. Even the original designer wont be able to make sense of it in the future, let alone anyone else.

[LukeW]

Drawing up schematics is just like writing code.

You can write code that is functional and technically correct... but without good structure, layout, indentation, commenting, documentation, sensible names of variables and functions etc. it is a pain in the arse if you have to take somebody else's code and work on it and maintain it.

Drawing up schematics is exactly the same. You can have a technically correct, working schematic which is a pain in the arse to read and follow and debug, if the layout is not tidy and neat and well documented.