Bill Shen's QuadSer 4-port UART module is subject to spurious interrupts due to floating inputs on unused input pins.
Hector Peraza discovered this while porting his terminal driver for RSX280 to support this board.
The "work-around"/fix is to add some small resistors across the receiver inputs on each channel to ground. This lowers the input impedance and shunts induced noise to ground, and does not interfere with normal operation.
Both Hector and I used 10Kohm 0.1W radial lead resistors - you'll need four of these, and they're soldered directly to the rear of the QuadSer Rev1 module that has already been modified for the Z280RC.
Hector provided this Photo of the QuadSer modification with the resistors in place (highlighted).
