Units Are needed to follow a protocol so the interrupt-request (IRQ) lines could be shared. The PCI bus features 4 interrupt lines, INTA# by way of INTD#, all of which can be obtained to every product. As many as eight PCI gadgets share the same IRQ line (INTINA# by INTINH#) https://nathanlabsadvisory.com/achieve-pci-dss-compliance-in-the-usa-secure-your-business-today/