[sdiy] Question about tri-state buffers and encoders

Donald Tillman don at till.com
Sun Sep 10 23:42:37 CEST 2023

Hey David, 

Leaving a CMOS logic input open is an error.  The sooper high input impedance means that the pin could accumulate charge and float to any level; low, high, in between, perhaps even beyond the rails.

For the CD4532, many of the inputs go through two inverters in a row, so it's entirely possible that it found a state where, say, "a" and "not a" where both low, and it broke the priority logic.

  -- Don
Donald Tillman, Palo Alto, California

> On Sep 10, 2023, at 12:47 PM, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
> Hello SDIY Team,
> Here's a question for the digitally inclined.  First a bit of context:
> I recently designed a circuit that uses CD4532 8-to-3 encoders (with a 4071 to make a 16-to-4 encoder, as per the datasheet).  The inputs are being driven by 74HC126 tri-state buffers.
> The circuit wasn't working -- all of the encoder outputs were staying high.  I had three of the encoder inputs unconnected.  I connected them to ground, and now the circuit works.
> However, I'm still a bit confused.  During normal operation, many of the tri-state buffers are in the Z state -- essentially disconnected from the output buss.  I have not put pull-down resistors on those outputs, so it seems to me that the ones that are off are looking like an open circuit to the encoder input.
> My first question is, why is the circuit working at all?  Is there something special about a tri-state outputs such that they don't require being pulled down to ground not to look like an open circuit to a CMOS input?
> My second question is, even though the circuit works with just the unconnected encoder inputs grounded, should I install pull-down resistors to ground onto all of the buffer outputs anyway, just to be safe?
> Addendum: I built one of these for a Wiggler, with a different layout but the same circuitry, and it worked fine, even though the three 4532 inputs were disconnected.  Is it going to stop working at some point?  The difference was that the connections were through wires for that build, whereas the connections were from a PCB on this current build.  Otherwise, the circuits themselves were identical.  I guess I just got lucky with that first build.
> Thanks in advance for any responses,
> Dave Dixon
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