EXFO FTB-500 (Window XP) - Manual (Page 280)

Troubleshooting
270 FTB-500
LED Indicator Color Codes
LED Indicator Color Codes
The LED indicators can convey different message depending on their
colors and state.
LED Color and Status Message
Power

Green

Green,
blinking

Unit on.

Unit in Standby mode.
Battery (external
power connected)

Green

Green,
blinking

Yellow,
blinking

Fully charged

Charging

Module using or could be using more
power than what is available from
batteries. Do not disconnect AC power
while this module is in use.
A blinking yellow LED takes precedence
over a blinking green one, so when AC
power is connected, even if the LED is
blinking yellow, batteries are probably
charging (depending on conditions).