Asus Z8NA-D6 - Manual (Page 52)

2-32 Chapter 2: Hardware information
4. USB connector (10-1 pin USB34; A-Type USB5)
These connectors are for USB 2.0 ports. Connect the USB module cables
to connectors USB34, then install the modules to a slot opening at the
back of the system chassis. These USB connectors comply with USB 2.0
specification that supports up to 480 Mbps connection speed.
5. Serial port connector (10-1 pin COM2)
This connector is for a serial (COM) port. Connect the serial port module
cable to this connector, then install the module to a slot opening at the back
of the system chassis.