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).

http://www.captain.at/electronics/atmel-programmer/atmega16-programmer.png

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

 

If they are connected by a wire then there is no potential difference between the two grounds.

 

The schematic shows them joined. Look at pin 2 of IC2. They should be joined for that particular application.

 

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.

 

Thanks for the replies. I guess I was just trying to make it too complicated