Robert the Parallel cable is added to Long Exposure modded SPC900NC's Webcams. Without the mod the camera works only as a Webcam. IE it takes video rather than single shots like a Digital SLR Camera.
If you only have a single USB cable comming out of your Camera then it is not a long exposure camera but is still great for imaging planets and also for Guiding.
A Modded SPC900nc can do all of that with the addition of being able to take single shots. These shots can be held open for long periods of time like on a Digital SLR Camera that allows more light to buid up so you can take shots of things like Nebula and deep space objects. It's the same as taking night shots with a DSLR camera,. You need to open the appeture for longer..
How does this effect PHD you might ask? Well if you are able to use the long exposure feature of your camera (if it has the extra lead and been modded) you will be able to see more stars. As the camera can let more light in or stare for longer, you will get more light into your camera and hence see more stars. This is an advantage when guiding in area's that have limited stars in the area.
don't worry if you camera is not modded as you can still use it to Guide, but modded Long Exposure is a little better, but does cost more to purchase or to have modded. They can be found on E-bay and it's a local guy that does them
Regards
Martin