Jinx,

In addition to what Richard Prosser wrote, some other things to consider 
might be:

1) Look at the way it's done with things like hot-swap CompactPCI 
boards.  They have longer fingers on the power pins to ensure they get 
contact first before other signal pins.

2) Assess how input/output lines are going to work when a new board is 
plugged in. It will take some time to power up and initialize it's I/O 
lines. Is there a potential problem here? Can signals be isolated with 
diodes or resistors? Open collector?

3) You probably don't want to have a power switch on your plug in board.
If you plug the board in with it's power off via a switch, the I/O pins 
may be subject to latch-up behavior as they may be powered up from the 
external bus.

4) I once saw a system that could have hundreds of boards attached to 
it. They could be plugged in at any time.  It was a very simple board; 
it only read 3 switch closures and reported them when polled.  The 
system worked by tugging on the power line and signaling via current. 
The benefit was 2 wire operation.  Address switches made each board 
distinct.  Comm protocol allowed for polling the boards one at a time. 
Of course, if your system is busier, that approach may not work.

Good luck!
MD
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