I have an Arduino mega controling a led grid of 512 5050 leds. (Adafruit Neotrellis boards)
I have a 5v 10 amp Power supply connected to 12 points on the grid 6 positive 6 ground ( it says ground on the board) The grid not connected to arduino.
And arduino usb connected and just SDA and SCL pins connected, no volt o ground
There seems to be aproblem with the ground connection.
With that set up nothing happens, bt if a plug a cable from the board to ground on the arduino then it works....
Seems like the SDA and SCL pins that you connected to the led grid require a common ground. Basically, I2C communication requires a common ground between all of the connected modules because the current passed between the communication lines needs to get back to the source-the board that sent it in the first place. By adding a common ground between the Arduino and the led grid, you essentially completed the circuit allowing I2C communication between the boards. You can read more about this in PerryBebbington's post.