Asus RS520-X5/PS8 - Manual (Page 53)

ASUS RS520-X5/PS8 4-11
8. 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.
The serial port module is purchased separately.
7. USB connector (10-1 pin USB34)
This connector is for USB 2.0 ports. Connect the USB module cable to
this connector, then install the module to a slot opening at the back of the
system chassis. This USB connector complies with USB 2.0 specification that
supports up to 480 Mbps connection speed.
The USB port module is purchased separately.