On Thursday 10 August 2017 17:10:17 Les Newell wrote:

> Hi Gene,
>
> > I've had a heck of a time with noise when using a 7i90 interface,
> > noise so bad it destroyed several 7i90's by punching thru the 3.3
> > volt circuitry.
>
> That's some serious noise. Do you know where it is coming from? A near
> field probe is great for finding noise sources.  Wrap a dozen turn of
> wire around an insulating former, say around 2" diameter then wrap
> some tape around it to hold it in place. Plug the loop into your
> oscilloscope, crank up the gain and wave it around any suspicious
> areas. It is usually pretty easy to home in on the noise. With smaller
> probes you can even trace the noise to a particular area of a circuit
> board.

My probe was a ferrite cored standup choke of a few microhenries, and it 
found lots of noise about the pair of switchers driving the steppers, 
more noise than the stepper drivers were making. Analog supplies for the 
steppers helped a lot, but moving the pi and interfacing out of the 
power electronics box, combined with the 7i42TA's and its all happier 
than a clam. I put a 20amp corcom brick wall in the line power to the 
pi, and another in the 256 volt feed to the vfd.  Probably overkill but 
it did help, and the vfd doesn't seem to care.  Its in the line feed, 
not the motor load.

> Noise can come from unexpected places. For instance my inverter 
> has a built in switch mode power supply to power it's logic, as do
> most inverters. On mine that power supply stuffs more noise back down
> the supply than the inverter's main drive section!
>
> Les
>
>
>
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Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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