Asus RS300-H8-PS12 - Manual (Page 68)

Chapter 4: Motherboard information 4-20
2. Baseboard Management Controller LED (BMC_LED1)
The green heartbeat LED blinks per second to indicate that the ASMB7 is working
normally.
The heartbeat LED functions only when you install the ASUS ASMB7 Management card
4.4 Onboard LEDs
1. Standby Power LED (SB_PWR1)
The motherboard comes with a standby power LED. The green LED lights up to
indicate that the standby mode of the system is ON, in sleep mode, or in soft-off mode.
This is a reminder that you should shut down the system and unplug the power cable
before removing or plugging in any motherboard component. The illustration below
shows the location of the onboard LED.