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

[DJ Delorie <dj@xxxxxxxxxxx>]

[I posted this on s.e.design, but all of the people who responded were
too stupid to know what "fun challenge" meant, and tried to talk me
out of the *design* of the tester, ignoring the purpose of the
posting.  So, I'm copying it here, because people here tend to be more
intelligent than that, and might enjoy a mental challenge without
missing the whole point of the posting.]

I had this idea, and toyed with it a while (including some computer
simulations), but didn't go very far with it.  I thought I'd post it
here in case anyone wants a brain teaser for the weekend.

Consider this tool: a cat-5 cable tester.

Goal: To make a tester that works entirely from the user's side,
without needing batteries at the far end.

Circuit:

+-----+
|     | P0/AD0     R1                      R11
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P1/AD0     R2                      R12     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P2/AD2     R3                      R13     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P3/AD3     R4                      R14     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|  uP | P0/AD0     R5                      R15     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P1/AD0     R6                      R16     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P2/AD2     R7                      R17     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     | P3/AD3     R8                      R18     |
|     |----------\/\/\--< <--------> >----\/\/\ ---+
|     |                     cable
+-----+

The idea is that each GPIO pin can either drive high, drive low, or
measure the voltage present.  I.e. you're creating a programmable
resistor divider network.  So you can set up various combinations of
resistors being driven from various combinations of voltages, and use
the A/D converters to measure the voltage, and compare with what you
"think" it should be.

The challenge: Select values of R1..R18 such that you can accurately
detect the following cable errors:

        * shorts between any N (2..8) conductors.
        * opens in any one or more conductor.
        * swapped (or N>2 miswired) conductors.

Bonus points for minimizing the number of A/D bits you need.

My idea was to provide eight red/green or RGB leds on the uP side to
indicate the status of each conductor (green=OK, other
combinations=various error conditions), with a serial or usb port for
diagnostics to a laptop.  I have a cable tester now, but the battery
is at one end and the lights are at the other.


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