<html><head><meta http-equiv="content-type" content="text/html; charset=us-ascii"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Hey David, <div><br></div><div>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.<div><br></div><div>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.</div><div><br></div><div> -- Don<br><div><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div style="color: rgb(0, 0, 0); font-family: Menlo; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">--<br>Donald Tillman, Palo Alto, California<br>https://www.till.com</div></div>
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<div><br><blockquote type="cite"><div>On Sep 10, 2023, at 12:47 PM, David G Dixon via Synth-diy <synth-diy@synth-diy.org> wrote:</div><br class="Apple-interchange-newline"><div>
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<div><span class="404142519-10092023">Hello SDIY Team,</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">Here's a question for the digitally
inclined. First a bit of context:</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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.</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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.</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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.</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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?</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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?</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">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.</span></div>
<div><span class="404142519-10092023"></span> </div>
<div><span class="404142519-10092023">Thanks in advance for any
responses,</span></div>
<div><span class="404142519-10092023">Dave Dixon</span></div></div></div></blockquote></div></div></div></body></html>