Re: gEDA-user: Fun challenge: cat-5 cable tester

[DJ Delorie <dj@xxxxxxxxxxx>]

> Is there a typo in your diagram?  You list P0/AD0 twice (same for

Perhaps.  Eight A/D converters on eight GPIO pins.

> My first thought, drifting off to sleep last night, was that all of
> the resistors should probably have the same value.  My second thought
> was that R11-R18 will probably end up wanting to be half of the values
> of R1-8... or is that double? :-)

My thought was that R1-8 (the ones on the uP side) need only be big
enough to protect against shorts.  Say, 220 ohms or so (there will
always be two in series with a short, Voh-Vol about 5v, short limited
to 10mA.

The resistors on the far end should be different values, so you can
tell which lines they're connected to.  Choosing the right values so
that you can generate useful voltages is the tricky bit.

> You mention some computer simulations... do you already know the
> solution to this puzzle?

It was just software that let me plug in different resistor values,
and calculated all the possible dividor networks.  It then noted the
two closest resulting measurements.  From that, I got down to a 14 bit
DAC, which IMHO is a poor quality result, and doesn't take into
consideration the various shortcuts you can take in the logic, for
example, test for shorts and opens first, then knowing there are none,
you can simplify the tests for swapped conductors.


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