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I thought that parallel pots was superior that USB! Am I wrong?I am asking to learn,the only experience I have is with parallel ports.

Can't really look at it in better or worse terms because the main reason for USB connection is to allow use of external motion controller which have much higher spec than parallel port. So it's not really fair to compare to PP.
The parallel port is limited to 25Khz for stable operation, if you have really good PC setup then possibly can reach 45Khz maximum.
Also the quality of pulse generation on external motion controller is often much higher quality which allows much faster and Smoother operation. Hence why often get called "Smooth Stepper" after the Original Mach3 based USB motion controller made by Warp9 in US.
The other main difference is the number of I/O it allows. The PP is much more limited.

So if you don't require high feed rates with high Micro stepping rates and can live with 5 inputs and 2 Outputs then PP will work fine.

USB will allow motion controller with more I/O and Speed and connection via laptop etc but does have some issues with reliabilty regards dropping connections and inteference from outside sources. It's also funerable to reliabilty with Windows getting in the way so needs careful setup of PC to stop interuptions while using machine.

If you want better solution then go with Ethernet based controller like Cslabs or UC300eth.