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Interfacing two separate boards - ground question

  1. Mar 5, 2012 #1
    Hi all,

    I am trying to interface two separate boards. the power supplies of both the boards are different. One board would act like an output board in the sense it gives either 0V or 24V and the other acts as input board accepting 24V or 0V (any voltage in between as well). Now main doubt is the two boards are having their own grounds. so i need to short the grounds of both the boards so that each can understand that 24V of one board is the same as the 24v of another board? if i connect the signals alone it will not serve the purpose? Even though it is a simple question i request your inputs on this. Is it true for any system?

    thanks and regards,
  2. jcsd
  3. Mar 5, 2012 #2
    It is hard to give an answer until I know a lot more about the system. It is not a simple question. First, what are the ground reference of the two board? Are they in the same card cage or in different modules all together.

    If they are both on the same ground where the common of the two power supply are tied together at one point, then it should not be too bad. I don't know the detail of your system, most of the case you connect the two board solidly together with the interface cable....be it ribbon, coax etc. Instead you put toroid at their power feed.
  4. Mar 5, 2012 #3
    Thank you for the reply.

    The system is I have a Digital input output card with the PCI slot which sits in the computer PCI slot and I can control the voltage outputs from this card using PC (0V or 24V). The output of this would go to another entirely different embedded system (micro controller based) which can accept 24V or 0V (any voltage in between as well, i use a voltage divider circuit to bring it to TTL level and give to micro controller). The PC based I/O card has got ground signal and the micro controller board has got a ground. Should i short this two ground pins to achieve the common reference point?

    Thanks and Regards,
  5. Mar 5, 2012 #4
    In most case, yes. That's how most interface do, just connect the two ground solidly together.
  6. Mar 5, 2012 #5


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    Staff: Mentor

    As yungman says, connecting the grounds well is necessary.

    But your bigger problem is avoiding latchup of driven inputs. When you have a system with multiple power supplies, you need to be careful about driving a logic high signal into a part of the system that is not powered up yet. If you drive a high signal into an unpowered circuit, and then power that circuit up, you can latch up that input circuit, which can cause the circuit to burn up.

    Either the separate power supplies have to be guaranteed to power up together, or the signals that are driving from one system to the other have to be held low until the other system is powered up, or there has to be enough series impedance in the signal line to limit the current to below the SCR latchup current sensitivity of the other system's input circuits.

    More reading on latchup:


  7. Mar 5, 2012 #6
    Yes, the question did not come up. Our products always had to pass CE which required to have pull up pull down transorb protection on every single signal line. This also protect the input. If the power fail at the input side, if the output from the other board drive high, it will turn on the transorb diode and conduct to the Vcc of the board that is off. Usually it is enough to prevent the output from going high OR if the drive is strong enough, it will pull the Vcc up also!!! But either way, nothing burnt in our experience.
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