Ground1 != Ground2, right?

Thread Starter


Joined May 18, 2007
Basic question, I think. I'm having trouble understanding how this circuit is operational (with reference to the grounds).

There is an external voltage source applied (bottom right) where the green, external GND (let's call it GND-in) is connected directly to the GND that connects to the parallel port (call it GND-p).

Is voltage between GND-in and GND-p zero, always? Could GND-in be lower than GND-p, making a short between the two and frying my computer's parallel port?

Thanks. Still on the steep part of the learning curve, I guess :)


Joined Apr 27, 2007
In you case, the ground is all the same. You have to connect all grounds. If you don't connect the circuit's ground to the port's ground, the input and output signals will not always be understood (because the circuit is "floating" in relation to the computer's port). So, it is essencial to connect all grounds, since in the schematics there is only one type of ground represented.