Ground1 != Ground2, right?

Discussion in 'The Projects Forum' started by Pootworm, May 18, 2007.

  1. Pootworm

    Thread Starter Member

    May 18, 2007
    29
    0
    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 :)
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    If they are connected by a wire then there is no potential difference between the two grounds.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    The schematic shows them joined. Look at pin 2 of IC2. They should be joined for that particular application.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    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.
     
  5. Pootworm

    Thread Starter Member

    May 18, 2007
    29
    0
    Thanks for the replies. I guess I was just trying to make it too complicated :)
     
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