Asus RS300-E8-RS4 - Manual (Page 61)

ASUS RS300-E8 Series 4-13
9. Serial port connectors (10-1 pin COM1/COM2)
These connectors are for the serial COM ports. Connect the serial port
module cable to one of these connectors, then install the module to a slot
opening at the back of the system chassis.
10. Power Supply SMBus connector (5-pin PSUSMB1)
This connector allows you to connect SMBus (System Management Bus) to
the power supply unit to read PSU information. Devices communicate with an
SMBus host and/or other SMBus devices using the SMBus interface.
This connector functions only when you install the ASUS ASMB7.