A Net Optics PCI Port Aggregator tap is a port aggregator tap. It combines the TX side of whatever’s plugged into port A with port B into a single output on port C. If power to the tap fails (i.e., the PC is off), the tap still passes traffic. In a real deployment, one tap interface would go to the router and the other tap interface to the firewall. The third tap interface connects to the sensor monitoring traffic passing between router and firewall.”]
Source: https://taosecurity.blogspot.com/2005/05/tap-on-pci-card-those-of-you-whove.html