I can't remember how i resolved a similar issue like this with this chip previously, but I know I've managed it.

I've just looked at the project again and the receiving unit never sends data without a request for it to be sent, so the code doesn't need to expect the arrival of random data. I've just tested it using HSERIN and HSEROUT and it looks like I'll be able to work with the native Pbasic commands instead.

I like having control of interrupts, usually, but this time it's not important to me