What are external interrupts?
Standard on each Origin 2000, Origin 3000, Onyx2, and Onyx 3000 system are two external interrupt ports (one input and one output), which are designed to be connected to external equipment. The interface to these lines is provided by special device files found in /dev/external_int (see ei(7)). This interface allows separate machines to send and receive interrupts over a dedicated wire for inter-machine synchronization. Physically, the ports are female 3-conductor 1/8 inch audio jacks identical to those found on portable stereo headphones.