Units are necessary to observe a protocol so that the interrupt-request (IRQ) traces could be shared. The PCI bus includes four interrupt strains, INTA# via INTD#, all of which are offered to each product. Around 8 PCI devices share the exact same IRQ line (INTINA# by means of INTINH#) in https://nathanlabsadvisory.com/achieve-pci-dss-compliance-in-the-usa-secure-your-business-today/