Four outgoing external interrupt lines appear on the back panel of the computer. You can control them individually, creating pulses or simply asserting and deasserting the lines.
Two input jacks for external interrupts are provided. Either of these jacks can cause an interrupt, but you cannot distinguish which jack caused a given interrupt. The interrupt is level-triggered, not edge-triggered.
For details of the use and programming of external interrupts, see the IRIX Device Driver Programmer's Guide, and see the ei(7) reference page. You can use the external interrupt as the time base for the Frame Scheduler. In that case, the Frame Scheduler manages the external interrupts for you. (See "Selecting a Time Base".)