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I bought a SainSmart clone board which is compatible with the Arduino Uno R3. Alongside each of the standard IO pins, there is a row of 3 additional pins (male). They are arranged in columns, marked S, V, and G.

They can be seen on this image:

Photo of SainSmart Uno R3 clone board

Most of them are immediately above the SainSmart and UNO logos. The SVG pins corresponding to the analog pins can be seen immediately below the microprocessor.

What are these pins for? Is this a standard format for some applications, or is it something unique to the SainSmart?

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SVG = Signal, Voltage, Ground.

The Signal pin will carry the actual output, which may be high or low at any given time. It's basically just a male version of the corresponding standard GPIO pin. The Voltage pin will always be high (which can be 5v or 3.3v on this board, depending on the output level switch). The Ground pin is exactly what the name suggests -- it's connected to the board's ground.

This obviously means there is a lot of duplication. Why have two signal pins? And why bother having so many Voltage and Ground pins if they're all the same anyway?

The reason is simply convenience.

If you have a standalone external component (such as a motor), you would normally have to run 3 separate wires to it. With the regularly-spaced SVG pins on the board, you can instead use a 3-way cable with a suitable female header block. You can run that to an equivalent set of 3 pins on the external component. That means you are able to attach/detach the component by plugging/unplugging (effectively) a single cable.

I don't think the SVG idea is a 'standard' as such, largely because (in this form) it's only really of benefit to people who are experimenting or prototyping (as opposed to designing a more permanent system). It seems to be fairly popular among hobbyists and enthusiasts though. You just need to watch out for components which may put their equivalent SVG pins in a different order.

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    Servos usually have the same pinout so I guess the other benefit is that you can easily plug them in. imagesco.com/servo/connectors.jpg – sachleen Feb 19 '14 at 18:11
  • Great point. I'd meant to mention servos, but obviously forgot! – Peter Bloomfield Feb 19 '14 at 20:04
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    This is also useful for plugging potentiometer to analog pins. If you plug the external pot pins to V & G and the middle pin to S, then you have an instant voltage divider. – Ricardo Feb 28 '14 at 2:23
  • I should point out that it appears the V pin in this configuration is not powerful enough to drive a standard servo. This is unfortunate, because it is really convenient to plug a servo directly into this header. It seems to work at first, but eventually the board will go into an unresponsive state, presumably because it can't drive that much current, or the voltage is fluctuating. I'm not exactly sure why, but driving the servo off of the Arduino header 5v pin seems to work just fine. Kind of a bummer. – Brian Genisio Aug 10 '14 at 1:03
  • It's not really a good idea to power a servo of of any pin on an Arduino, as those would either have too high a voltage or be sourced from the logic regulator, and as servos are notoriously power hungry and electrically noise, it is preferable that they not share the same regulator as logic. – Chris Stratton Aug 12 '14 at 20:08

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