What I found was the chip control lines were floating when in the low state.
What do you want to say???
If the arduino digital output pins do not sink to ground when in the "low" state,
Ah but they do.
1) Unpredictable outputs may occurr with a 4051 type chip (this is the chip I have experimented with) when the input control lines (A, B, C, and D in the post) are either not driven at ~5v input or not connected to ground.
2) If the arduino digital output pins do not sink to ground when in the "low" state, then the control lines to the multiplex chip my "float" and produce undesired results.
3) The solution provided might just be a workaround for the real problem
So, what is causing the problem
zoomkat you are obvious so clever why can you not work it out? You are clever enough to use a chip in a way it was never intended to be used so why are you not clever enought to work out why it will not work?
See Mike's answer.
Oh come off it zoomcat. Your "peanut gallery heckler"s have probably forgotten more about that chip than you know.
So you think "your usual "..and this is not the way to use an analogue multiplexer." is some sort of conspiracy by the knowledgeable and experienced to withhold arcane knowledge from newbies? Electronics is not magic. That chip didn't just appear last week. People have been using it for decades, and it seems doubtful that someone who is "new to this chip" is going to find some mysterious new feature or function.
The presenting symptoms of dysfunction aren't enough proof for you? The argument is pointless.(I'm not going to waste my time proving that the sun appears to rise in the east, either.)
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