USB JTAG software

I have only ever used a parallel JTAG, which was fine, met my needs OK.
No real reason to do anything.

But I found a USB JTAG, it has a prolific serial chip on it.
It was for an Arduino project.
I think it should be useable for cable modems too.

I installed the prolific chip driver, it works in device manager.

I have a couple of DOCSIS 3 units I want to play with, a Motorola, a Cisco, a Scientific Atlanta. Assuming they have JTAG, which I have not yet investigated.

What software for reflashing modems would be usable?
I tried the BlackCat USB software, that doesn't seem to know it's there.
I think the usbjtag.com software only works with their product, which this is not.

Any ideas?