Equipment are needed to follow a protocol so the interrupt-request (IRQ) lines could be shared. The PCI bus incorporates 4 interrupt traces, INTA# by INTD#, all of which are available to each product. As much as 8 PCI devices share the identical IRQ line (INTINA# by INTINH#) in APIC-enabled x86 https://nathanlabsadvisory.com/iso-22301-business-continuity/