Units are needed to stick to a protocol so that the interrupt-request (IRQ) strains can be shared. The PCI bus incorporates four interrupt traces, INTA# by INTD#, all of which can be found to each unit. As much as eight PCI devices share the exact same IRQ line (INTINA# through https://nathanlabsadvisory.com/iso-55001-2014-certification-it-asset-management/