Problems with Xilinx Parallel III Cable

<EvalXX@gmail.com> wrote in message 
news:1175195472.611147.280310@n59g2000hsh.googlegroups.com...
> On Mar 28, 6:55 pm, "John_H" <newsgr...@johnhandwork.com> wrote:
>> <jid...@hotmail.com> wrote in message
>>
>> news:1175098335.158323.203250@n59g2000hsh.googlegroups.com...
>>
>> The newer Xilinx programming cables (Parallel Cable IV, Platform Cable 
>> USB)
>> use comparators rather than simple buffers to establish proper voltage
>
>
> Where can I get these kind of comparators? What about multiple
> voltages? Any sample circuits I can use as a reference?
>
> Thanks
> - E.

Almost any kind of comparators?
Take a look at the Platform Cable USB Product Specification

http://direct.xilinx.com/bvdocs/publications/ds300.pdf

pages 11-12 for an idea what Xilinx does to get the voltage compliance and 
well-defined threshold values.  If you substitute the 5V LPT buffers for the 
CPLD, you can pretty much cut & paste your way from a Parallel-III to a 
Parallel-IV style cable (though you'd still refer to it in SW as a 
Paralell-III).

I should reiterate that having your VCC act as both a VREF and source for a 
DC-DC converter is a solid way to design for the 5V LPT interface and the 
lower-voltage JTAG interface.