Devices Are needed to observe a protocol so the interrupt-request (IRQ) traces could be shared. The PCI bus consists of 4 interrupt lines, INTA# via INTD#, all of which are available to every machine. As much as 8 PCI units share exactly the same IRQ line (INTINA# through INTINH#) in https://nathanlabsadvisory.com/iso-20000-1-it-services/