Quote from: BkkCoins on July 03, 2013, 11:19:56 AM

It was an issue with interrupt handling in the PIC. But the solution creates a timing sensitive condition where I've had to make the USB have priority and this could cause result bytes to be misread. I'm looking for a way to handle this now. Result bytes need to be handled in < 8uS and originally I had this block all interrupts to catch them (32uS blackout), but this is enough to throw the USB into fits, so I need a way they can co-operate but guarantee the result bytes get moved.

I guess your problem is blocking of interrupts while handling an interrupt.
Are you taking too much time to handle an interrupt ?
Can you create a low-level assembly interrupt routine that only gets one byte of data, stores it in a circular buffer and returns again ?
This low level routine can then handle both usb and result bytes input.
Higher level routines can then poll for data in the circular buffer.