this is a simple non-rt busy-wait loop waits for ACK on the parallel port ,then it will toggle pins D0-D7 to produce an ACK. Again this runs on the second linux box and is connected to the MiniRTL box via PLIP-cable. if you drive the load high on the computer running lpt_irq you can see that the MiniRTL system will time out sooner or later since the non-rt program lpt_irq will fail to deliver the ACK in the required timeout defined in rt_irq_gen.c.
To prevent this behavior you can use rectangle.o on the second box, which is a rt-program that does basically the same as lpt_irq just it does it in real-time and thus driving the load high on the PC will not influence the behavior of the MiniRTL system. This deterministic behavior is what its all about and this simple example shows the difference most drastic.