or has anyone tried using a regular USB to UART adapter?
One user modified an old Block Erupter USB (those small things that plug straight into a USB port) to work with a Block Erupter Prisma;
[DIY] Tube/Prisma USB Adapter from USB BlockerupterHe notes:
You should theoretically be able to use any CP2102 USB-UART witha 1K pullup resistor from 3.3V to RXD in this fashion.
YMMV
Probably worth pointing out that someone else followed those steps and got a generic CP2102 adapter working. Looking at tubes the reason the 1K resistor is needed is that the controller on the tube board connects to the UART via a 7407, which is an open drain buffer.
https://bitcointalk.org/index.php?topic=865503--
novak