two digital circuits - logic not recognised

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Joined Oct 13, 2009
I have two logic circuits that need to pass information from one to the other... it is possible that one will be running off of a different power supply than another. most of the time the operational voltages will be the same 12VDC. sometimes this is not the case and usually in these cases the information comes in as garbage. when the DC minus of both supplies are connected, everything works perfectly. what is the accepted name of the issue that is resolved by using a common ground?