Units Are needed to observe a protocol so that the interrupt-request (IRQ) strains might be shared. The PCI bus consists of 4 interrupt lines, INTA# by INTD#, all of which are available to every device. As many as 8 PCI gadgets share the same IRQ line (INTINA# by way of https://nathanlabsadvisory.com/ensuring-secure-payment-processing-with-pci-dss-compliance-certification-in-the-usa/