Asus P9X79 PRO - Manual (Page 51)

ASUS P9X79 PRO 2-31
Chapter 2
6. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF) port(s).
Connect the S/PDIF Out module cable to this connector, then install the module to a
slot opening at the back of the system chassis.
The S/PDIF module is purchased separately.