[Coco] Faster CoCo for George... Was: Re: OT: Gramical nitpiking >My CC3>CC4

[Chuck Youse]

On Sep 7, 2008, at 3:29 PM, Tim Fadden wrote:

> I am no electronic engineer by any stretch of the imagination, but  
> would opto-isolators/couplers work for the FPGA/coco-perf's  
> connections? It would make it electrically isolated from each other  
> for sure. I don't have a clue how it would be done, but I do know  
> they work. :-)
>
> Tim Fadden
>

Good thinking, but not an appropriate solution.  Optical  coupling is  
generally used to isolate electro-mechanical devices from the rest of  
a system -- this is because of the high-voltage/high-current  
transients that coils tend to generate which will fry digital  
components.

In this case, it's a simple CMOS -> TTL level translation that's  
required.  In the case of outputs from the FPGA (assuming 3.3V IO  
pads, which is generally the norm), they can drive TTL logic  
directly, though you have to watch the fan-out.  It's the inputs that  
matter, as CMOS inputs have higher Vih requirements (usually 0.7VCC,  
which is much higher than TTL's 2.0V) and they can't stomach more  
than about 3.6V without getting angry.

74LVT or 74LVX -- I forget which family exactly -- fit this bill  
perfectly.

C.