Asus P5Q3 Deluxe/WiFi-AP @n - Manual (Page 64)

2-36 Chapter 2: Hardware information
13. Digital audio connector (4-1 pin SPDIF, for ASUS HDMI VGA card)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF)
port(s). If you are using ASUS HDMI-equipped graphics card, connect the
HDMI card to this connector with a S/PDIF out cable.
The ASUS HDMI-equipped graphics card and S/PDIF out cable are purchased
separately.
14. Serial port connector (10-1 pin COM1)
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.