On Sat, Jun 21, 2008 at 08:21:38PM -0700, Marcel wrote:
> Wow, to think a PIC is associated with such cool science experiments as 
> the hadron collider...  congratulations!

Thanks! I'm doing my best to get these two cool pieces of hardware
together, although I suspect somewhere between those gigameters of wire
there are already quite a few PICs hiding.

But even if my current evil schemes come into fruition, this is not for
the current phase of the LHC (which is all but starting operations), but
for its upgrade, due in about 10 years.

> What you need to do for connecting depends on two things here mainly: 
> speed and environmental noise.

I expect both to be relatively low. To take advantage of the full
accuracy of the sensors they cannot be sampled at more than 40Hz.
Assuming roughly 10 bytes per sample and five sensors, this yields about
20kbps, well within the low range of both protocols' specs.

I expect EMI levels to be low, though I don't really know. Asking
around, they tell me that it's very quiet unless one of the big cranes
along the ceiling moves - they have badly isolated electrical motors.
My rule of thumb is that it will have to work on my improvized workbench
first. I'm sure it's more noisy than anything we'll encounter there.

> I don't know how much capacitance per 
> meter you should expect with CAT-5E cable but if your acquisition rates 
> are low enough, it should not affect things too much. Check to see if 
> the input pins you will be using are schmitt trigger types; these will 
> be far more tolerant of slow rise times.

Google suggests values around 50pF/m, which is why I said a total of
400pF is probably a bit too close.

Schmidt triggers are provided on both sides of the loop, but I guess
that is moot once I have line drivers - now they will need to trigger
properly.

Can you give me a hint about slew rate? Would I want them as large as
possible to compensate for the 'sluggishness' of the wire, or do I want
them controlled at some fixed fraction of the clock cycle to prevent
possible overshoots or ringing?

> Even low capacitance cable can have problems in high noise environments 
> so this cannot be answered by someone who isn't on site, I think.  We 
> have all seen photos of "atom smashers" with enormous electromagnets 
> pulsing tremendous currents and such.  So after deciding on the first 
> question of how fast you need to run, the next question is to decide how 
> much shielding you will need.

LOL! ATLAS, the experiment I'm working on, indeed contains the largest
magnet ever built (in terms of flux), but it doesn't pulse by a long
shot. It just sits there with its 20kA flowing through its
superconducting coils (8 of them, it's a toroid). I never saw it in
action (it's quite dangerous, I'd have to leave my glasses behind) but I
doubt it even hums.

And the tests I'm trying to join will be far away from it, in some quiet
rural side beam that will never see all that action, just a low rate of
pions at a few GeV. Still, you are absolutely correct about having to be
on site. After everything works, I'll build the most indecipherable UI,
to ensure myself a plane ticket to the Alps.

> Good luck and absolutely understand you must report back on how it all 
> goes!!!

I will, though unless you're into it, you'll probably find these tests
very boring. To see how much, google for 'SLHC thin gap chamber
upgrade', click on a few PDFs, and imagine a little PIC patiently
reporting the alignment of the gas chambers relative to the beam and to
each other while a group of physicists scratch their head, trying to
figure out why they can't manage to improve their spatial resolution.

Cheers,
Yair.
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