Re: gEDA-user: terminators

[gene glick <carzrgr8@xxxxxxxxxxxxx>]

DJ Delorie wrote:
>> have a little mismatch to your trace, maybe a little ringing.  Beats 
>> loading it with an inductive stub.
> 
> What about just putting a 100 ohm series resistor right after the via,
> on the stub?  At least it will isolate the sdram lines from the stubs.
You're in a tough spot.  You have 7.5 ns cycle time.  With 100 ohm, it 
only takes 75pF to consume the entire timing budget.  The rise time of 
the network will completely distort the readings at the LA.  I think 
your ribbon cable is going to have at least that much capacitance (and 
probably a bunch of ugly inductance to ruin your day).

This is why the mictor connector exists.  Placing such a small stub on 
the line, with the LA pod having very low input capacitance, and the pod 
has buffers right at the mictor end of things, the signal does not 
degrade.  The PCI guys struggled with this very problem 10 years ago.

I'm willing to bet the input capacitance of the LA is high enough to 
make more problems too.

> 
>> What voltage are you running?
> 
> 3.3v

They don't mention edge rates in the data sheet, but tpd is 1.7 nS worst 
case. I think you will get some idea of what is going on your bus - 
albeit with some timing distortion.

Here's another idea.  Instead of the LA, use a scope - assuming you can 
deal with looking at just a couple of signals at a time.  Put the scope 
into 50-ohm mode. Connect a 50-ohm coax with BNC on one end to the 
scope.  Cut the end of the coax to expose the center and the shield. 
Solder one end of a 1K resistor to the center - the other end gets 
soldered to your bus where you want to probe. Solder the shield to the 
board as close as possible to the place where the resistor is soldered. 
  You should get a very faithful reproduction of the signal (with the 
possible exception of amplitude distortion - maybe).

I've heard of people using SMA connectors for this, and buying cool 
resistor divider probes for their scope.  But for the diy guy, just do 
what I said in the last paragraph.  It works really well - amazingly 
well actually.  In fact, you could just stub out some soldering test 
pads on the traces, and sprinkle some grounds around.  Then just solder 
the resistor wherever you want to probe.  Not very elegant - but it works.
> 
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