Tristate Buffer Output if Input is High-Z?

[Tony Duell]

> 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?

An analogue switch IC is a similar device, and those do exist. The 
problem is finding one that will switch quickly enough (I would guess in 
a few nanoseconds) for this application. 

Also remmeber that analogue switches don't buffer the signal, you don't 
get an increased drive current if you use one.

-tony