Drive pins to ground instead of using real ground

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If your signal return path current needs to take this path, then the current needs to go through the FPGA I/O pin in the wrong direction.

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There's no "wrong direction" for a CMOS output stage.

Of course the return current will create a voltage drop across the output transistor and also ground bounce according to its dI/dt. In so far it's not a perfect ground. But in terms of signal integrity of the ribbon cable still better than nothing.