[sdiy] Question about tri-state buffers and encoders

David Manley dlmanley at sonic.net
Tue Sep 12 16:36:17 CEST 2023


One thing that hasn't been mentioned, or maybe I missed it:  a tri-state 
buffer feeding into a high impedance cmos input is essentially a 
sample/hold.  After the tri-state has been driven once, the 'floating' 
node will maintain it's voltage for some time.

-Dave

On 9/12/23 1:19 AM, Roman Sowa wrote:
> 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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