Tristate Buffer Output if Input is High-Z?

[Peter C. Wallace]

On Wed, 28 Sep 2005, Jeff Walther wrote:

>> Date: Tue, 27 Sep 2005 07:54:24 -0700
>> From: Eric J Korpela <korpela at ssl.berkeley.edu>
>> Subject: Re: Tristate Buffer Output if Input is High-Z?
>
>> On 9/26/05, Jeff Walther <trag at io.com> wrote:
>>> 
>>>  If a tristate buffer is enabled by its control line, but the input to
>>>  the buffer is at high-Z is there a typical output?
>> 
>> 
>> If there is it probably varies between parts. If you need a defined output
>> for tristated inputs you should pull the inputs up or down.
>
> Thanks to everyone for all the replies.
>
> It sounds like the tristate buffers don't do exactly what I would like, 
> although I think I can make it work with more logic in front of the control 
> line for the buffer.
>
> However, doing a bit more research, it looks like what I really need is a 
> Transmission Gate.   The important difference is that a transmission gate 
> will pass the input regardless of its state, so L passes L, H passes H and Z 
> passes as Z.   The disadvantage is that if you have a noisy signal, the TG 
> doesn't clean it up at all the way a TB will.
>
> The catch is that I cannot find one listed anywhere as a part that one can 
> actually buy.  Are transmission gates purchasable parts?  Or are they just 
> something they discussed in my VLSI textbook?



Google for something called a quick-switch, It is basically a single 
NMOS transistor transmission gate. Pericom ( and others) make them.

Peter Wallace