I've replaced the motherboard in a PC with a small SBC which has an
intel 9-pin front panel connector[1] rather than just a bunch of header
pins for LEDs and switches like a normal motherboard, the connector pins
are labelled in the board's own manual[2] as

1 HD_LED_P
2 PWR_P/SLP_N
3 HD_LED_N
4 PWR_N/SLP_P
5 RST_SW_N
6 PWR_SW_P
7 RST_SW_P
8 PWR_SW_N
9 UNUSED

so the power LED is between pins 2 and 4, and rather than a single
on/off LED, it offers the ability to use a bi-colour LED.

The way they're labelled PWR+/SLP- and PWR-/SLP+ I assumed it required a
2-pin LED with red/green wired in inverse parallel, such as

https://www.maplin.co.uk/p/twopin-qy83e

I bought one, it lights up green when power is on, but not red when
power is off (by reversing the connections I can make it light up red
when on, but still not green when off, so it isn't a case of the LED
being half dead).

Presumably this means in reality it requires a 3-pin LED, such as

https://www.maplin.co.uk/p/threepin-cj53h

And that I need to split the ground from e.g. the hard disk LED cathode
pin to feed the power LED's common cathode, the labelling feels wrong
though ... would you have read it the way I did initially?



[1] http://www.formfactors.org/developer%5Cspecs%5Ca2928604.pdf
[2] http://download.udoo.org/files/UDOO_X86/Doc/UDOO_X86_MANUAL.pdf