> However, each GPIO goes to one AND-input (and one red LED) each, and
> from the same input it goes through the cable in question to test, and
> the opposite end of the cable goes to the second AND-input. The
> AND-output goes to a green LED.
> 
> This means as soon as one GPIO goes high, one AND-input and one red
> LED goes high, and if the cable is functional, the other end makes the
> second AND-input high, making the output high, making the green LED
> lit.
> 
> I'm not sure if I'm making sense, but I hope so.

The design looks fine to me. Do add pull-down resistors to the second
AND inputs though, if you haven't already.

My suggestion: whip out a multimeter (or just hotwire some LEDs) and
test the PIC outputs. You might have miswired the AND gates, or maybe
the chip is defective, or the wire to test is wired wrong (in which case
your tester seems to be working perfectly). Testing the midpoint (the
PIC outputs) will allow you to rule your program out, if it's not the
problem.

With the behavior you described, it might be that the cable-to-test is
miswired. If there are no pull-downs on the AND gate inputs, you can get
all sorts of funky behavior due to varying charge on the FET gates when
inputs are floating (especially if a long but unterminated wire is
connected). Once you do add pull-downs, if the behavior persists, it
might be the test wire is just connected improperly, or the AND gates
are (assuming the PIC tests out fine as above). If the PIC doesn't put
out the proper states, then either there's someone competing with it
(e.g. the AND gate outputs are wired to the PIC outputs), or there's a
code problem (As others have mentioned, check analog mode. Doublecheck
TRIS too.)


-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

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