The Heartbeat, battery and AC LED's are used to indicate the operational status of the B520 Auxiliary Power Supply.
Flash 1x/ sec:
Normal State: Indicates normal operation state.
Trouble State: Power exists, but no communication.
Trouble State: Indicates a trouble condition exists.
LED Trouble State: Module is not powered, or some other trouble condition prohibits the module from controlling the heartbeat LED.
OFF for 3 sec, then flash pattern:
Firmware version* Firmware Version: For example, if the version is 3.2.1, the LED pulses 3 times and pauses for 1 sec, pulses 2 times and pauses for 1 sec, and then pulses 1 time. After flashing the Firmware version, the heartbeat LED stays OFF for 3 sec before indicating the module state.