PCB Assembly Services - Screaming Circuits: Single Node Nets


Single Node Nets

I've read various opinions on schematic technique regarding the illustration of nets and nodes. Some folks want to route as much as is possible with net lines and some folks think it's clearer to use a lot of node symbols and avoid a mess of parallel lines.

For example, in the following schematic, I barely route anything with net lines. Normally, I wouldn't use this approach, but with an MCU that does almost nothing but blink a bunch of direct drive LEDs, it seemed easier and clearer in this specific case to avoid showing all of those nets.

Node style
I usually route the nets in the schematic except where clarity dictates otherwise - like when two nodes are on opposite side so the sheet. Of course, when I'm drawing a multi-sheet schematic, I have to use node symbols for nets jumping sheet to sheet. Really, I don't see much inherently wrong with node symbols as opposed to net lines. It really depends on which style is more clear.

One danger I do find with the use of node symbols is the ease at which I can end up with a single node net, or a broken net. Certainly, a DRC check will usually find such an error, but sometimes a broken net won't be found by that DRC.

I sometimes vary the names of my power nets. I have caught myself with a "3V", as in the schematic above, a "3.3V", "V+", "VBUS", "VUSB" and a few others. I usually have good reason for each, but if I copy a schematic clip from a project with "3V" power rail to one with a "+3V" rail, I have to take extra care. Using node symbols only, the DRC may very well see every node connected to something else despite the fact that my "3V" circuit and "+3V" power section are isolated.

When running net lines, your CAD software will notify you when you try to connect the two sections with different net names and it will be much more difficult to head over to layout with that problem still in place.

The real moral of this story is that node symbols are a common point of error and deserve some extra time during design review - don't just rely on the automated DRC check.

Duane Benson
Delicious Ricotta Cheese does not count

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Reviewing the netlist in a text editor can be helpful in finding errors like your broken net (3V and +3V), or if each net in a bus has the same number of connections.

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