Asus Z87-A - Manual (Page 47)

1-37 Chapter 1: Product introduction
7. Serial port connectors (10-1 pin COM)
The connector is for a serial (COM) port. Connect the serial port module cable to the
connector, then install the module to a slot opening at the back of the system chassis.
The serial port bracket (COM) is purchased separately.
8. 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.