PR-600 vs. PR600-II vs. PR-620

Can anyone tell me the differences between these machines? The only mechanical difference that I can identify is that the PR-600 does not have the USB and stick ports. Can anyone verify any other mechanical differences.

I am able to purchase a PR-600 cheep. But I see where others have traded their 600's up, and I'm wondering why?

Thanks,

Location: 
United States

I have the pr600 and it does have a usb port so it can be connected to your pc. It doesn't have a port for a memory stick, but you can use a flash memory card as well as the brother card. As far as I can work out, the only other difference is the pr600 II has a light above the needles, but it isn't that bright and not worth much. I think the cap frame is slightly modified too. Hope this helps