USB port is for viewing and controlling guide camera with software like
PHD...on a laptop.
The guide software determines any guide error and sends a correction back to the guide camera (via USB) which then outputs the appropriate Ra+. Ra-..etc earth pulse to the mounts ST4 (RJ11). PHD auto-calibrates and thus it can determine which way is N/S/E/W...no need to worry which way you put in the guide camera. This will cost around $350Aust.
That's generally how most "cheap" auto-guiders work.
However...
the original ST4 was a standalone guider with a control panel that didn't need a laptop nor guide software. The same with the
newer SG4 guider but this doesn't even need a control panel...it's just the head unit.
But...these stand alone solutions are more expensive (around $1800 Aust for the SG4) and I guess most people figure if they are imaging then you'll need a laptop in any case.
A
cheaper guiding solution is "pulse guide". In this you use something like a web-cam as a camera (aka a Philips SPC900NC, $90 Aust) which plugs in via USB.
Again you use PHD but this time you select ASCOM pulse guide..that is you control the mount via ascom (either serial control or USB control..or whatever to your mount) and PHD adds drive "pulses" to make the mount move faster or slower in the desired direction (correcting for guiding errors). This works very well and is a lot cheaper. But of course..you must be able to control your mount via ASCOM!
All this said...I think most people like the simplicity of a guide camera and ST4...well I do
.
Hope this is not too long winded!
..more reading
here by Craig Stark.
edit:
You can get
USB to ST4 converters to use web-cams as an ST4. There are also a
new generation of cheap
standalone guide camera out nowdays. Meh!...not my cup of tea..although the
TV guider is a treat. A few IIS'ers use this one was for sale recently!