On Sat, Jun 21, 2008 at 10:24:37PM -0400, Bob Ammerman wrote:
> 
> SPI can be more easily extended than I2C because none of the lines are 
> bi-directional.
> You can use whatever line-driver chips you want to create a robust link.

I guess line drivers are indeed unavoidable - the maximum total current
the sensors can source is less than a mA, a far cry from the PICs.

Since I will be using CAT-5, I guess driving two of its twisted pairs
through differential transceivers such as those for RS-485/422 would keep
SPI's data lines happy.

That would leave 4 wires for power, clock and slave select, unless the
clock would want a twisted pair of its own, in which case I'll have to
get clever with feeding the sensor.

> Of course if you're in a noisy environment (like near a bunch of hardrons 
> colliding into one-another) you might need some pretty special line-drivers. 
> I would be tempted to do something that was 'current-loop' based to have a 
> very low impedence.
> 
> Or, you could even use an optical interface if noise is a big problem.

Near the accelerator and the interaction points, I believe most communications
outside heavily shielded Faraday boxes is indeed optical, but the test
stand we'll be using will only see a relatively low current test beam,
so I don't expect the noise levels to be much higher than in a high tech
office. I just want to have reasonable margins.

> In any case, it would just be a wire from the point of view of the software.

Unless I chicken out and throw another small heap of hardware at the
problem to implement some Error detecting encoding scheme. :-) My only
cross check right now is to see that the vector sum of the three axes
is reasonably close to 1g, but the Devil, as they say, is in the LSBs.

> -- Bob Ammerman
> RAm Systems

Thanks,
Yair.
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