On Fri, 27 Jul 2018 16:38:58 +0100
David Higton  wrote:

> 3) If two outputs are connected together, but they are at opposite
> logic states, it's possible that enough current will flow to damage
> one or the other pin.  Thus, in software terms, you'd want to ensure
> that all pins that are going to be connected together have been
> configured to appropriate signal directions.  If in any doubt,
> make the Pi's pins inputs before plugging up.

 Alternatively a small resistor (1K will probably do) can be placed
in the lines then even if two connected pins are both outputs for a short
while they won't blow each other up. Lesson learned doing the Torch Z80
board - the data on the ports on both sides said on reset they were inputs,
what it failed to mention was that between power up and reset they might be
outputs for just long enough to break them, resistors solved the problem.

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