[sdiy] Question about tri-state buffers and encoders

David G Dixon dixon at mail.ubc.ca
Sun Sep 10 21:47:02 CEST 2023


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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