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I've been told that there's a design rule of thumb that whenever you have a "bus" providing power and ground (for example, in an array of PWM outputs) that the ground connection is placed nearest the edge.

What is the reasoning behind this?

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    \$\begingroup\$ Don't design by heresay. There are some reasons NOT to put the ground at the edge too. The "I've heard" or "everyone knows" rules are the worst out there, and need to be examined very very carefully before basing any design on them. \$\endgroup\$ – Olin Lathrop Nov 9 '12 at 13:10
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    \$\begingroup\$ Yep... that's why I'm asking here! \$\endgroup\$ – Mark Harrison Nov 10 '12 at 18:26
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Ground ring around the periphery as a sort of EMI shield is what I was taught, but I understand that in a quarter century, the validity of that purpose may have worn off.

Pure speculation alert: One possible benefit of keeping the neutral rail consistently at the outer periphery of a board is that accidental contact between two boards near the edges would not cause catastrophic short circuits.

There is no reference I can quote to validate this speculation, though.

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A practical reason: If you had the signals bus at the outer edge, they can be more difficult to route.

In addition, it does become a little bit of a ground wall at the outer edge, which is usually a good thing.

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I was always taught that it is because the ground rail of a circuit normally runs around the outskirts of a board to help preventing any external noise. This is especially important in amplification circuits for obvious reasons but I suspect nowdays with surface mount and multi layer boards its probably just an old habit/rule of thumb with no real benefit.

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  • \$\begingroup\$ This does not answer the question, but it would be a good comment. \$\endgroup\$ – Trygve Laugstøl Nov 9 '12 at 11:09
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Lots of interesting reasons from everyone! I worked on a robotics team for a number of years and we always followed that rule largely so that a stray wire that was touching the grounded frame was slightly less likely to hit a positive lead. Essentially it was a small protection against having grounded things short out because the high-current positive rails were shielded by the ground pins. With our team it was never a hard and fast rule though. We did it when possible and convenient.

The extra benefit of that "rule" is that in a three-pin system, putting the positive rail in the middle and having a current-limiting resistor on the signal line makes it more difficult to damage the system by plugging it it backwards. We actually set up one system so that it was ground-positive-ground to introduce the "can't plug it in backwards" property because it was a $400 part we were plugging in!

With PWM lines it's also just a matter of practicality. That will match how commercial servos are wired hence why you normally just make it match so that you don't rewire all the servos yourself.

Anyway, no idea if this also is passed down due to EMI reasons, but that was why we did it.

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