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There's no "wrong direction" for a CMOS output stage.
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True, otherwise we would not have BusSwitch components. However, the explanation was intended as a "thought exercise" to aid in understanding, eg., that the return signal current needed to travel in the wrong direction relative to the CMOS driver logic low current path (which was the intended current path for that CMOS gate).
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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.
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Do you really think this would work?
If the two ground planes on the two boards were not connected, and the cable ground pins were all connected to logic low signals from the FPGA, you'd have a bunch of drivers driving a common ground (the cable and external board), this would not cause a "driver conflict", since they're all driving ground. All the return currents from signals going from the board to the FPGA would return through various paths to the FPGA logic level low outputs.
The "ground" connection would probably be pretty noisy, but I guess it might work ... its not something I'd want to use for anything other than a quick hack though!
Cheers,
Dave
Honored Contributor
