Jinx,

On Wed, 23 Feb 2005 23:11:10 +1300, Jinx wrote:

> > 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.
> 
> Hmmm, easy to build into the edge connector. The Vss finger could be
> a little longer still

Yes, if you can use edge connectors you can do things like:

Have the 0V (I can't be doing with this Vss, Vdd stuff :-) fingers at say 100% length
Have the +5V fingers at 75% length
Have all the other fingers at 50% length
Have a slot in the board and a key in the socket to ensure correct orientation and alignment
Arrange the connection physically to guarantee the right sequence even if the board is plugged-in slightly 
crooked, so say 0V at each end of the connector, with +5V next-in at each end, or with +5V in the middle

This means that you have a reference established before the power comes in, and no control/data connections 
are made until the whole voltage setup is connected.  That way any protection components (pull-up or down, 
diodes, zeners) have something to work with and will work as designed.
 
Back in the good old days of Z80s with backplane busses, I seem to remember using 74LS245s for the bus 
interfaces - as I remember they are tri-state, and it should be possible to keep them in high-impedance state 
with pull-whichever resistors until the PIC is ready to talk to someone else.

Remember that until the PIC has powered-up, stabilised, and the program has started running, you don't have 
much more than Ohm's law working for you!  :-)

Cheers,


Howard Winter
St.Albans, England


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