[sdiy] Question about tri-state buffers and encoders

David G Dixon dixon at mail.ubc.ca
Wed Sep 13 19:35:33 CEST 2023


Exactly.

Plus, I cannot use pullups in my case, because some of the busses themselves
will be in the Hi-Z state (all of their feeding buffers will be off).  If I
pulled those inactive busses up, they would feed erroneous high logic to
their encoder inputs.  They have to be pulled down.

I have another question about all of this.  Once I grounded the top three
encoder inputs, the circuit worked perfectly.  Given that this is a
"priority" encoder, and all of the logical inputs are connected to each
other in some fashion, does grounding the top three inputs (or even the top
input alone) serve the same purpose as pulling down (or up) every input
individually?

(Note: I originally sent this message only to Roman by mistake.)
 

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Roman
Sowa
Sent: Tuesday, September 12, 2023 1:19 AM
To: Jay Schwichtenberg; synth-diy at synth-diy.org
Subject: Re: [sdiy] Question about tri-state buffers and encoders

[CAUTION: Non-UBC Email]

What's the difference? If there are pull-ups, you're "fighting the 
resistors" when signal goes low as well. Same load. Especially since all 
logic families invented during last 40 years have symetrcical current 
capacity in H and L state.

Roman

W dniu 2023-09-11 o 21:55, Jay Schwichtenberg via Synth-diy pisze:
> As mentioned tri-state is typically used for buses. You don't want to 
> use pull-downs on those. You use pull-ups to keep the bus from floating. 
> If you use pull-downs you're fighting the resistors when the signals go 
> high. If you use a high resistance with pull-downs with CMOS it will 
> probably work. If you use a lower resistance then you may not be able to 
> pull the signal high when needed.
> 
> Jay S.
> 
> On 9/10/2023 12:47 PM, David G Dixon via Synth-diy wrote:
>> 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?
>>
> 
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