[kamocat]

The little green LED will still flash regardless of whether the DIO module has 12v power or not. It will simply flash much dimmer without the external power.
Just swapping out components is a good way to break things. (The Digital Sidecar supposedly has reverse-voltage protection, but who knows if it was implemented?)

However, you are correct I was making assumptions about the functionality of other parts. A good test for the digital sidecar would be to to use a digital input or output. (If you want to test the motor controllers that way, a 1.75ms pulse should be about 50% full speed)