Femtobuck wiring clairifcation

[jfbauer]

In the [hookup guide for the [femtobuck , it says

Note: When using an external supply for the Femtobuck, the grounds of the two boards must be connected!

Is that referring to the Arduino ground and V- on the femtobuck (external power supply ground), or just ground between 2 or more femtobuck boards?

I’ll probably be using 1 femtobuck with probably 3 LEDs in series. When looking at the LEDs in series diagram in the hookup guide, at first I assumed the two black wires that cross were not connected, but then I saw that note I quoted above and now think they should be connected. But that would mean that V- and D- would also always be connected. But if that is the case, why would they both need to exist?

In general, when looking at the wiring diagrams, should I assume that 2 lines of the same color that cross mean that they should be connected?](https://www.sparkfun.com/products/13716)](FemtoBuck Constant Current LED Driver Hookup Guide v13 - SparkFun Learn)

[TS-Chris]

Is that referring to the Arduino ground and V- on the femtobuck (external power supply ground), or just ground between 2 or more femtobuck boards?

That’s saying the ground for your external power supply and the ground for the Arduino need to be connected together.

The crossed wires don’t connect where they cross, but they are actually connected together inside the board. When crossing wires connect there’s usually a dot on the crossing point to let you know they connect.

[jfbauer]

I get that the Arduino GND and external power supply ground need to be connected. But since they are connected inside the femtobuck, I should not need another ground-to-ground connection of my own.