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I was just looking at the power connector for a SATA optical drive, and noticed that it had fifteen pins!

I then marveled at the data connector next to it, which has a measly seven pins.

I know that there are only physically three pins needed for the power connection:

  • +12v DC
  • +5v DC
  • Ground

Why did they decide to use a fifteen-pin power connector that's twice the width of the data connector?

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3 Answers

up vote 11 down vote accepted

Here's the SATA data & power pinouts.

Remember, SATA is a serial bus. This means data transfer only needs two paths -- TX (transmit) and RX (receive). In the case of SATA, there's actually 2 pins for each (a TX+ and TX-, and a RX+ and RX-); this is called twisted pair and (just like in twisted pair Ethernet) allows for longer wire runs with less noise from other wires. The other data pins are for ground, which also assist with noise elimination. So SATA doesn't need more data pins.

Power, on the other hand, supples 3.3v, 5v, 12v, and Ground. Not to mention extra pins (not present in all connectors) for hotplugging, activity indication, and staggered spinup. As to why so many? Wikipedia again:

Each voltage transmits through three pins ganged together, because the small contacts by themselves cannot supply sufficient current for some devices. (Each pin should be able to provide 1.5 A.)

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...and yet all of the power adapters I see don't handle the 3.3v pin. It must be considered de facto optional. – Broam Mar 23 '10 at 14:04
@Broam: there's 3x 3.3v pins. are they all missing? they might be (should be) missing on molex-to-SATA power adapters; those won't have a 3.3v lead, just 5v/12v – quack quixote Mar 23 '10 at 16:56
I was referring to the Molex(TM) adapters, yes. The only ones I see that handle 3.3v come straight from a PSU. – Broam Mar 23 '10 at 17:53
@Broam: yep. you'd need additional hardware to provide 3.3v from a 5v & 12v source. for most of what you'd need the molex adapters for (SATA optical drives or hard drives for an older computer), the devices can get away with not using the 3.3v. newer low-power devices may not work properly with those power plugs. – quack quixote Mar 23 '10 at 18:55
3.3v isn't really a requirement yet I have only seen it on a 1.8" SSD so far although obviously we are moving that way as component sizes decrease. You only won't have it if you are using a Molex adapter. – PeteT Jun 3 '10 at 8:32
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up vote 4 down vote

http://en.wikipedia.org/wiki/Sata#Power_supply - specifically

A third voltage is supplied, 3.3 V, in addition to the traditional 5 V and 12 V.

and

Each voltage transmits through three pins ganged together, because the small contacts by themselves cannot supply sufficient current for some devices. (Each pin should be able to provide 1.5 A.)

So that is nine pins needed for power, though very few (if any) drives use the 3.3V lines, plus a few for ground.

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up vote 1 down vote

http://en.wikipedia.org/wiki/Serial_ATA#Power_supply explains this nicely.

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