On Sunday 01 January 2017 10:45:53 Peter C. Wallace wrote:
> On Sun, 1 Jan 2017, Gene Heskett wrote:
> > Date: Sun, 1 Jan 2017 10:33:44 -0500
> > From: Gene Heskett <ghesk...@shentel.net>
> > Reply-To: "Enhanced Machine Controller (EMC)"
> > <emc-users@lists.sourceforge.net>
> > To: emc-users@lists.sourceforge.net
> > Subject: Re: [Emc-users] question on emi filters
> >
> > On Sunday 01 January 2017 05:54:05 Bertho Stultiens wrote:
> >> On 12/31/2016 09:18 PM, Gene Heskett wrote:
> >> [snip]
> >>
> >>>> However, if the reference ground is not equal at both ends of the
> >>>> signal line, then you are in trouble. This is why you would use a
> >>>> balanced line (differential), which is not absolutely referenced
> >>>> to ground, but switches on the differential (and you can clamp
> >>>> the lines).
> >>>
> >>> Do you know of a 4 wire to 8 wire and back interface that can
> >>> function at 32 megabaud and doesn't cost 5 grand+? Neither do I.
> >>> :) Laser diodes and detectors that could handle 2x the video speed
> >>> needed to hit an HDTV transmitter would be required, times 4 to do
> >>> it optically.
> >>>
> >>> There are capacitatively coupled chips I have seen the
> >>> announcements for, intended to steal some of the jobs the MOC
> >>> chips are doing, but no clue as to their useable bandwidth. I'll
> >>> see what google says.
> >>>
> >>> A paper by Silabs
> >>> <https://www.silabs.com/Support%20Documents/TechnicalDocs/CMOS-Dig
> >>>it al-Isolators-WP.pdf> says the are using a nominally 10 megahertz
> >>> carrier in a cmos circuit, about 30x too slow for this.
> >>>
> >>> Searching further, TI has a family of them, 3 channels one way, 1
> >>> the other, would only need one, at 3.49 in 1k lots, claims 100
> >>> megahertz bandwidth. I'll see how much power it needs, and what
> >>> the small qty price might be. Not available, so I've ordered
> >>> samples of 2 variations, one of which is inverting but I don't see
> >>> which is which. Its an SOIC package, dunno if I could hack a pcb
> >>> for that.
> >>>
> >>> Perhaps this might be the better method?
> >>
> >> Actually, I do know how to do it cheaper than $5k. The Silicon Labs
> >> isolator chips are rated up to 150 Mb/s, so that is not a real
> >> issue.
> >
> > Nowhere in that paper I just read does it claim 150 MHz, only that
> > the coupleing clock is 10 MHz.
> >
> >> A LVDS driver and receiver can be had for $1..2. Then you need some
> >> support logic and power management (power isolation). All in all, a
> >> three channel system on CAT3 (CAT5 patch-) cabling would be in the
> >> order of $20..30 in components. Add some overhead and we are at
> >> about $50 (disregarding my hours of designing the circuit).
> >
> > This is a $6 chip in our qty's, and needs 4 ea .1 bypass caps on a
> > pcb. An rj45 could be used, but would run up the cost. Solder pads
> > wouldn't bither me a bot, I'm a CET. Since this would normally be a
> > fixed installation, it seems to me a 6 wire ribbon which could steal
> > power from the devices on each side of it would do nicely on a pcb
> > no bigger that an airmail stamp, including an isolated tab for a
> > firm mechanical mount. Put two solder pads on the power connections
> > and you have used all 8 wires in a 9" piece of cat5. And you would
> > have 5 kv of isolation. My problem is in fabbing a footprint for an
> > SOIC chip on dbl sided pcb on my milling machine. I do have the
> > etching mills, I bought a 10 pack for the last project. And once the
> > code is written, more pcb's are just time & worn bits if you dig
> > into the glass, the target is removing the copper without cutting
> > into the glass. Doable as long as the Z post is well lubricated.
> >
> > I think its worth investigating.
> >
> >> (isolator, see f.ex.
> >> http://www.mouser.com/ds/2/368/Si864x-51666.pdf)
> >>
> >> A proper digital balanced isolator is no rocket science and does
> >> not need to be too expensive. It just takes time to design, build
> >> and test.
> >>
> >>
> >> However, the real issue is the distance you can carry the signal
> >> due to propagation delay. If you use bidirectional SPI, then you
> >> have clock, data-out (MOSI) and data-in (MISO).
> >>
> >> At 32MHz clock you have a maximum of 15.6 ns of round-trip delay
> >> that can be tolerated on the data-in with respect to the clock
> >> (half-period). That translates into 1.5 meters(*) of cable
> >> (velocity at ~0.6c) and we ignore all the intermediate electronics'
> >> propagation delays. Adding in a buffer/driver (whatever type) at
> >> the cable ends introduces roughly 5..6 ns delay if you are lucky.
> >> That would reduce the effective length to under one meter.
> >>
> >> Not all is lost; If you send much more data than you receive, then
> >> you can still send at high speed where all signals are propagation
> >> balanced. Then, for reading, you reduce the SPI clock frequency
> >> considarably to take the round-trip propagation delay into account.
> >>
> >>
> >> (*) the signal-delay is measured round-trip which is twice the
> >> cable's length. The SPI master sends a clock, traverses the cable,
> >> causes a MISO change, traverses the cable, read by master.
> >
> > Well, in my case, the cable might be 8" long. The power to the pi
> > and the power to the 7i90, originate from the output terminals of a
> > separate 5v supply. But the pi has a ferrite choke on its power
> > lead that the 7i90 lacks. The 5v supply's ground terminal is taken
> > back to the common point bolt with a heavy braid. Now the
> > shield/ground return from the encoder cable, which is not grounded
> > at any other location, shows, depending on how I've hooked it up,
> > anywhere from 400 mv of noise at the x motor power supply switch
> > rate, to at one point over 5 volts p-p. So with the spindle stopped,
> > the encoders velocity outout shows anywhere up to a 3 digit speed
> > output, either direction. This noise, at a 17 kilohertz switching
> > rate, has less than 5ns rise and fall times. One thing I've not
> > tried yet is to ground the shielding on this cable to the encoder,
> > not to the 7i90, but to that common bolt. But the data error is 2
> > way, I've had the z motor, running along at 5 ipm, suddenly
> > accelerate to 60+ ipm, and run 6" before it unwinds and resumes its
> > commanded move, with this sudden move being in a random direction.
> >
> > The x motors supply came yesterday, so I have the mounting plate out
> > of the box, and am rigging a filter excised from a computer psu
> > mounted on one corner of it. Its input bypass to ground contact is
> > to this plate, and there is a huge ground lug about 3" away that I
> > will use for the common point.
> >
> > The noise at the 50 pin ground connection is easily 10x that on the
> > - power terminal of the 7i90, which tells be that ground at the i/o
> > connectors gets there by a rather circuitous route, so when I have
> > the supplies up and running, if the noise persists, I will move the
> > shield of that cable to the common bolt. Neither the pi, nor the
> > 7i90 actually has a grounded pad that could be used to give a good,
> > big wide braided common point to both. Frankly, its a noise magnet.
> > I have 2 of corcom's brick wall filters on order, rated at 20 amps.
> > I don't know what the rating is of the filters I have is, each is
> > equipted with a 3 amp 5x20 fuse. Based on the amprobe reading on my
> > milling machine , which is under 3 amps with the spindle off, and
> > perhaps 4 amps with it turning 500 revs and a brakeing belt wrapped
> > around the chuck making it do some work, if it blows the 3 amp, a 5
> > should hold it without blowing. The toroid wire gauge looks to be
> > able to do 6 or 7 amps without excessive heating. Its a test only,
> > to be replaced by the corcom filters when they arrive. I've left
> > room for one of those. The idea Bertho is to see if there is a
> > bulletproof method that will get rid of the noise.
> >
> > 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>
>
> before delving into the isolator and LVDS exotica,
>
> #1 are you using all 8 grounds on the flat cable to connect the 7I90
> to the RPI?
>
> You likely have a ground loop from I/O --> 7I90 --> RPI --?
>
> So anything you can do to lower the ground impedance from the 7I90 to
> the RPI (and raise the impedance in other part of the path) will help
>
>
> "The noise at the 50 pin ground connection is easily 10x that on the -
> power terminal of the 7i90, which tells be that ground at the i/o
> connectors gets there by a rather circuitous route"
>
> Nope, they all connect to a single uninterrupted ground plane on the
> 7I90
A ground plane that needs to be available for bonding purposes. Neither
the pi, nor the 7i90 is grounded by any means but their power cables as
they are screwed to 3/8" square legths of HDPE, which is in turn glued
to the panel. The pi has a ferrite choke on its power cable, but has a
long distance ground plane connection via the hdmi cable, currently a 12
footer to the monitor. Likewise a long usb cable to a switchable hub
because I currently have a wired usb keyboard. I have wireless keyboards
that I'd like to use, but the noise seems to be making the wireless
keyboards flaky too.
Then by capacitative coupling alone, and that cable is around 8 feet long
as it drops out of the bottom of the electronics box, runs around 5 feet
to get under the back of bed, then under the spindle casting pan, and
back up beside the drive belts to the encoder, so its a good antenna if
the whole lathe is bouncing with the the noise. I've tried a flat braid
connection from the common bolt to the lathes bed casting with zero
effect.
So far, only line filters have helped. So that is what I have under
construction now, with one other change, the supplies are now bolted to
the plate, where before they were glued, using a puddle of go-2, which
is NOT an electrical contact. At my working pace, I should be smarter
by dinner time. If these filters help. then the corcom brick walls
should be even better.
> Peter Wallace
> Mesa Electronics
>
> (\__/)
> (='.'=) This is Bunny. Copy and paste bunny into your
> (")_(") signature to help him gain world domination.
>
>
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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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