I would like to know the purpose of the LEDs on top of the Arduino WiFi Shield: L9/Link/Error/Data (left in the picture). What do they indicate?
The web page on the Arduino site for the board tells you in plain words what they mean:
On board indicators The shield contains a number of informational LEDs:
- L9 (yellow) : this is tied to digital pin 9
- LINK (green) : indicates a connection to a network
- ERROR (red) : indicates when there is a communication error
- DATA (blue) : indicates data being transmitted/received