(SPDIF)GPU to Mobo, 3 pins?

I have a 9800gt and I wanted to use the HDMI on the GPU to output video and audio at the same time so after some searching, I need to connect the GPU to the SPDIF port on the Mobo. The problem is, there are 3 pins on the SPDIF on the motherboard and only 2 pins on the cable supplied with the GPU. My motherboard is an ECS 6100pm and as you can see here, the SPDIF output has 3 pins. Which pins do I connect to?

What I can see on the motherboard is something like this Pin1 - Pin 2 - (no pin) - Pin 3

The manual to your motherboard ECS Web Site

Look at page 23

pin 1 SPDIF digital output
pin 2 5v analog Power
pin 3 Key (no pin)
pin 4 Ground

Update your system spec as it does not match your graphics card, I also need to know the manafacturer of your graphics card.

There, updated. I have yet to try it but I think Pin 1 and 4 sounds about right. thanks!