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'[EE]: serial comms between modules'
2001\05\15@062720 by Quentin

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Hi
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.

Thanks
Quentin

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2001\05\15@072341 by Michael Rigby-Jones

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RS232 with a pass through connector may be ok.  Each module would have to
have a unique address of course.  The advantage here would be even low end
PICs can do async comms easily.

Failing that I think I2C would be ok.  You can put pullups in each module,
as long as the combined pullup resistance dosen't get stupidly low.  You can
trade pullup resistance for maximum bus speed to some extent, so you could
put high value pullups in the modules (to ensure a reasonable minimum value)
and run the bus slower.  The disadvantage of I2C is that you will need a PIC
with an MSSP peripheral, and you will have to think about multi-master I2C
which isn't particularly easy AFAIK.

Regards

Mike

> {Original Message removed}

2001\05\15@081045 by Bob Ammerman

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I'd use conventional async comms with RS485, RS232 or opto-isolated
interfaces.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2001\05\15@090718 by Olin Lathrop

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> 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, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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