From the GHI documentation:
Digital input events rely on internal GPIO interrupts to work. On SITCore, these interrupts are only available on 16 pins at any given time, the pin number must be unique, over all of the available ports. For example: PA1 and PB1 cannot both be used as interrupts at the same time. However, PA1 and PB2, or even PA1 and PA2, can be used simultaneously.
Does the above statement also apply to use of an interrupt with a serial port? If so, I assume the PIN number that is “consumed” is for the receive pin?