Asus RS520-E6/RS8 - Manual (Page 56)

4-12 Chapter 4: Motherboard information
7. CPU and system fan connectors
(4-pin CPU_FAN1/2, REAR_FAN1/2, FRNT_FAN1/2/3/4)
The fan connectors support cooling fans of 350 mA–740 mA (8.88 W max.)
or a total of 3.15 A–6.66 A (53.28 W max.) at +12V. Connect the fan cables
to the fan connectors on the motherboard, making sure that the black wire of
each cable matches the ground pin of the connector.
DO NOT forget to connect the fan cables to the fan connectors. Insufficient air
flow inside the system may damage the motherboard components. These are
not jumpers! DO NOT place jumper caps on the fan connectors!
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.