> I am looking at ways to do low cost serial comms between a couple of
> boards:
> The boards will be within 1 meter from each other and must be "hot"
> swapable.
> Any ideas at the best type to use?
> Some thoughts of mine:
> Not sure if I2C would like the hot swapping because of bus state
> start/stop and it not floating when in use. Maybe I can make the last
> board I plug in wait in floating state.
> What about CAN? Problem I think of is that only very little high end
> PICs can do it. Same with USB (overkill, IMO).
> RS485, possible, but you need aditional tranceiver IC's.

Since you've only got a couple of boards, you can use point to point
communication between both of them instead of a multi-drop bus.  In that
case, it seems simplest to just tie the UARTs together.  The TX of one
drives the RX of the other and vice versa.  There is no need for driver
chips because you don't need to use the official RS-232 levels.  I would
still add a little zap protection where the signals go off board.  Just a
small series R for the outputs and maybe a small series R and zener to the
inputs.  Also, the inputs need pullups so that they float to the quiescent
state when the driver is unplugged.  Details depend on the speed you need
and your environment.

Hot swapping will glitch the line for a short time.  Tolerance of this needs
to be put into the software.  I would do the standard thing and wrap
everything into packets with checksums, etc.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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