alekseevic:
Hey LukeZ, does your solution work? I've recently bought a GSM shield from RS and I've just read this discussion, I've got the orange guy on my board and I'd like to minimize the risk.
Yeah, running power through VIN seems to work though I've only tested the shield by itself on the bench and not with the Arduino in the final application.
retrolefty:
One thing to be careful about when two or more sub-systems use separate DC voltage sources is what can happen if some of the voltage sources are turned on and some are turned off. Current can flow to/from input pins in unpowered parts and even pretty small amounts of current can damage clamping diodes and such.
I have not heard of this before, and would be somewhat skeptical but you have 16k posts! In my case the two boards should power on and off and roughly the same moment, but for the sake of understanding this better: let's say I have an Arduino with a digital input pin that reads 0 or 5v. The Arduino is off but there happens to be 5V on the input pin (common ground). This will damage my Arduino??
Or another situation is where you have a powered serial device hooked up to a non-powered Arduino, this would damage the serial pins on the Arduino?