Jamie Cox <jamie...@gmail.com
> My diodes appear to be okay. Here is what I am seeing. The problem affects only the switch register, SR1
> thru SR12. These are all in the "Row1" group. As long as one or more of the SR switches are off,
> everything is fine. However, as soon as all 12 of them are on, things get weird. Here is an example
> output from the new version of pidp-test:
Does it actually matter in which direction on the SR switches you
approach the 07777? ie. From your example it looks like you started
from the right most switch and moved to the left. Starting at the left
and moving right gives the same result? It already sounds like the
answer is yes, but I'm just confirming.
For a test (and because I don't think I actually tried it during my own
test) I turned all switches on one at a time from both the left and
right and from the middle working out. Mine worked as expected.
> I'm suspecting a sort of accumulation-of-load-and-tolerances problem, where I have enough drive for 11
> switches, but not 12. My three "1K Ohm" resistors range from 964 to 982 Ohms. The Row 1 resistor is 982
> Ohms. Which is certainly within 5%.
There's an enormous amount that I *don't* know about electronics. But
we're talking about a difference in current here of about 100 microamps
between the minimum and maximum values for your measured resistors. I
suppose that could be a possible trigger point (after all a fail has to be
triggered at some point) but I wouldn't leap to this conclusion first.
Maybe second... :-)
> Any ideas?
They're boring, and please don't ask me to explain how some of them
could cause the observed result (I don't know) but:
1) Solder bridge between the diodes. Along the chain of diodes the legs
are close, a bridge might connect two of them together. Bridges elsewhere.
2) Cold or missed solder connection on switch, diode, RPi connector, ...
3) Put the switches one toggle away from going nuts. Grab the body of
each switch while not touching the (flippy bit, paddle, toggle, GUI
whatever it's called). Wiggle firmly, but carefully. Try again after
putting it into "nuts" mode.
4) Repeat 3) above but instead try wiggling the paddle without flipping
5) Power down and remove your RPi. With your Ohmmeter check for
conductivity (red on left, black on right) across each SR diode. Make
sure there is NO conductivity with red on right and black on left.
I'm out of ideas.