I may be wrong (as usual), but the way I understand it, DISEQC commands are all digital commands modulated on a 22KHz carrier. The DISEQC-1.2 is a set of these commands used to control the motors. The motors have something like 50 preset positions stored in the motor, and the DiseqC commands just tell the motor to go to one of these preset positions. You can manually move the dish east or west when finding sats, then store the current position into one of the preset positions.
The way I understand it, USALS, is basically ONE additional diseqC command, which is basically a GOTO-X command, where the X is a number of degrees that the motor is to turn east or west from it's zero position. I think the resolution is about .1 degree. (1/16th rounded off to 1/10th). It is the receiver that does the USALS calculation. USALS is simpler for the beginner, since you don't have to search for the sats, you just tell the receiver what longitude you and the sat are at, and it does the rest. DISEQC commands require you to program each satellite, but has the advantage of having a little more control, allowing you to fine tune the sat positions.
If you have an oscilloscope, you can hook it up to your coax, and see the DISEQC commands being sent. Basically just a 22KHz signal that goes on and off to represent binary 1/0 conditions.
The part that I'm confused about is that there are 2 different types of DISEQC switches, and I don't know how they differ. One is called a mini tone burst or something like that, and the other is just plain DISEQC. The tone burst switches apparently use letters for the ports, like A,B, etc, while the regular DISEQC switches seem to use numbers, but I've found that either numbers or letters seem to work with both, so it doesn't seem to be a real difference.. I think my 4 port switch, which has given me nothing but trouble is a plain DISEQC, while my 2 port switch is the mini tone burst type, and it has been very reliable. I got both at
Sadoun, but Sadoun doesn't seem to sell the 2 port switches anymore. Too bad, because the 4 port switches seem to have problems.