Re: FPGA for PCI based servo control board

[Jon Elson <elson-at-pico-systems.com>]

ERIC KELLER wrote:

>Most boards you buy nowadays are what are called "universal" boards, i.e. they
>will work on both 5v and 3.3v.  You can tell if they have 2 slots in the
>motherboard connection fingers. As I understand it, they check to see if they
>are running at 3 or 5v and then start up as appropriate.  Almost all of the
>pc's I've seen have had 5v pci.  The dell I'm using has 3.3v 64bit slots and 5v
>32bit slots. 3.3v slots have the tab closer to the back of the computer.  The
>connector for 32 bit 3.3v slots are just the 5 v connectors backwards.
>
>I had thought about the issue of driving higher voltage outputs.  I've seen
>numerous app notes about this issue.  My experience is that 5v logic will not
>drive an opto22 module reliably, I always tie the upper control voltage to
>power and pull the lower leg to logic ground with the port.  Same scheme goes
>for opto isolators, assuming the fpga can sink adequate current. 
>  
>
Yes, the pull-up to +5 V on most families of logic circuits are not as 
strong
as the pull-down to 0 V.  This is well documented in the family design 
notes.
The Xiling Spartan makes a point of having symmetrical 24 mA source and
sink capability.  Otherwise, I always arrange for the logic to pull DOWN to
turn on the SSR.

Jon