[XPC Target] Targets syncronization

Referring to the article http://www.mathworks.it/support/solutions/en/data/1-1BR95/index.html?solution=1-1BR95, there's the possibility to syncronize two targets in a Master/Slave approach.

One master XPC target computer uses a digital output port to send interrupts which are received by the slave. The slave model is set to execute on interrupts arrival instead of using the system timer.

I don't have I/O cards but i read that i can use the parallel port to send the interrupts between the two computers. Why there's only the parallel port to do that? Why it's impossibile to use the RS-232 port?