Units are needed to comply with a protocol so which the interrupt-ask for (IRQ) strains is usually shared. The PCI bus includes 4 interrupt lines, INTA# through INTD#, all of which are available to each system. Approximately 8 PCI equipment share a similar IRQ line (INTINA# by way of INTINH#) https://nathanlabsadvisory.com/testing-for-compliance/