buffer_overfly:
The alternative power source must be connected in addition to the arduino pins on the right block. Thus, Arduino Vcc and GND should remain connected to the right group of pins, and also IN1, IN2, etc to Arduino's digital output pins, while the secondary power source must be connected to JD-VCC and GND pins in the left, after removing the jumper. I've read that in this case Arduino's GND should not be connected. This is actually nonsense, since both GND pins in the module are interconnected (you can see this by looking at the soldered paths in the back side) and I think it is bad practice to leave the arduino's Vcc floating in the module, instead of having a common GND for all the power sources. When using the alternative power source, it is possible to connect the right VCC in the module to just 3.3V.
Just to be clear, when using the optoisolation:
- Remove JD-VCC to VCC jumper
- Connect Arduino VCC to VCC
- Connect Arduino digital pins to IN1, IN2, etc.
- Alternate power source positive to JD-VCC
- Alternate power source negative to GND (any of the GND pins)
There's no need to connect the Arduino GND to the GND of the module -- and doing so will negate the optical isolation. The Arduino's VCC is connecting to the anodes of the optocouplers and the digital pins are acting as current sinks, connected to the cathodes of the optocouplers. It's functionally equivalent to connecting an LED to the Arduino VCC and a digital pin.