Ground is 0v. It's hard for me to explain, but it works both as sort of a
reference for whatever you use as +V supply (VCC) and as a return path...
if you have two power supplies, unless there are optocouplers involved, you
/want/ their grounds connected, as a general rule. That, however, does not
guarantee further trouble. Needing two supplies to power one device
typically results in one supply or the other trying to take on most of the
load, overloading, and blowing up -- leaving the second one to do the same
since it can't handle the full power of the circuitry....
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