In message 
          "NY"  wrote:

>Are the GPIO pins hot-pluggable, or will Nasty Things (*) happen if you
>connect/disconnect anything to them while the Pi is running?
>
>(*) Either at the software or the hardware level.

There's nothing in principle to prevent such hot plugging without
damage, but there are some significant practical problems that need
solving in order to give a guarantee in the worst case.

1) As another poster has pointed out, the voltage levels should be
compatible.

2) The 0V pin must make contact first, followed by the power supply
pin, and the signal pins must not make contact until power has been
established on the other board.  It's an unusual connector pair that
would sequence the pins like that.

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.

Dave

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