On Thursday 10 August 2017 12:09:45 Les Newell wrote:

> I have been chasing a noise problem on my lathe for some time now. I
> am using a 7i29 2kw drive and am getting a lot of noise on the
> encoders, even with filtering enabled at a pretty low frequency. I
> have tried just about everything I can think of. I made a simple
> near-field probe and did a bit of poking around. The 20kHz drive
> switching noise was strong pretty much everywhere. After trying
> various things I found a common mode choke on each motor output
> dropped the 20kHz noise down so it was pretty much undetectable apart
> from right next to the chokes. However I am still getting noise on the
> encoders. I have a spare encoder input on a 7i33 so I moved one of the
> motor encoders to that input. I am getting zero noise on that channel
> even though I extended the encoder cable to reach the 7i33 with 6
> individual wires instead of a screened cable. Even with filtering
> disabled I see no noise. Unfortunately I don't have any more spare
> encoder inputs for the other axis.
> Has anyone else had noise issues with 7i29 drives?
> Les
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 

Encoder outputs are fairly low frequency, and I first made a 
filter/voltage snubber circuit on a per wire basis, consisting of 1 
1N914 to clip the input from going more than .65 volts below ground, and 
5 more facing the other way to prevent anything in excess of nominally 
3.6 volts from getting to the 7i90. And I added a capacitor to 7i90 
ground to slow the rise times as the noise seemed to be well above 50 
MHz, and was nearly 30 volts P-P.  This made beautiful square waves out 
of the encoder signals at the 7i90 inputs that were not degraded even at 
a spindle speed of 1800 rpms, with a 60 tooth gear supplying the 
excitation .  However that didn't seem to be adequate for the steppers 
drives even when I added a 6th diode so the stepper could pull high 
enough to fully turn off its opto-isolation leds.  So I broke down and 
spent the $45 a copy for 3 7i42TA cards to put between the 7i90 and the 
real world, and threw away my cobbled up setup. These are 50 pin header 
for inputs, and do a great job.

I also bought a big alu amplifier box and moved all the control stuff to 
it, getting it out of all the power electronics noise. Everything but 
the local keyboard now Just Works(TM), and that keyboard problem is 
inherent to the architecture of the Pi's i/o, everything has to go thru 
an internal usb hub, except the gpio and video stuff.

Remote keyboards logged in work ok because they have the buffering effect 
of a tcp connection that despite going thru that usb2 pinhole serves as 
a handshaking buffer to interleave the traffic (the micro-sd card data 
also goes thru this data pin hole), but the local keyboard has no such 
buffer and keyboard events are lost at an unacceptable for machinery 
rate as the keyup is missed and the machine keeps on moving the axis 
last told to move.  Plays hell with editing gcode too, but there you can 
see the missed keydown as it never gets that key to geany. Conversely, 
the keyup is missed, so the last key typed turns into a 75 character 
string before you can hit another key to stop it, then of course it will 
miss the keyup for the correcting backspace, and erase 2 or more lines 
of gcode before you can get its attention again. 

One thing I haven't tried is to put 2 usb<->rj45 adapters, with the 
rj45's facing each other, between the keyboard dongle and the usb socket 
the dongle is plugged into. But the would need to be able to supply PoE 
too. Enough to run the 2nd adaptor and power the keyboard dongle.

I'm sitting around, waiting to see how pine's "rock64" works, which 
claims it fixes all the pi's shortcomings. It can also be had with more 
memory than the pi. If it works,this could be the pi killer we've been 
waiting for, running a bare path in the grass around the BBB. Shipping 
since the 1st of August, reviews as yet are rather sparse.

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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