Ok, this one has me stumped. We sell a product that hooks up to a PC via a serial port, and runs off a 15v power supply. Now, a lot of people are having problems using USB to serial adaptors to run these products and only certain brands of adaptor appear to work.

The only difference I can see in the adaptor is that the ones that work have thicker cables than the ones that don't work.

What could actually cause this? Is it some kind of problem with the power supply interfering with serial port on the device (they're practically adjacent)