Asus Z87-PLUS - Manual (Page 52)

1-36 Chapter 1: Product introduction
Chapter 1
4. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF) port. 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.