Units are required to stick to a protocol so that the interrupt-request (IRQ) lines may be shared. The PCI bus contains four interrupt lines, INTA# by way of INTD#, all of which can be found to each unit. As many as 8 PCI equipment share the exact same IRQ line https://nathanlabsadvisory.com/achieve-pci-dss-compliance-in-the-usa-secure-your-business-today/