Asus P9D-C/4L - Manual (Page 56)

2-36 Chapter 2: Hardware information
10. 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.
11. 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.