On Sunday 01 January 2017 11:29:45 Bertho Stultiens wrote: > On 01/01/2017 04:33 PM, Gene Heskett wrote: > > 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. > > Are /both/ 5V power supplies' GND connected to the common point?
I've had them floating, and connected, floating the voltages are in the 5v+ p-p range. Only 1, 5 volt supply and I have had the one I bought, and the one John sent me both in circuit for test, The 3amp from John is marginally worse but only maybe 10%, running both the pi and the 7i90. Without the motor supplies running, its quite low as long as the grnd terminal is connected to the common bolt, maybe 400 mv at about 45khz regardless of which small supply is hooked up. Bring up power to the motor supplies and its injecting enough noise on the SPI bus that it all goes in the crapper. Once I stopped everything but lcnc, and was walking around the tail end of it to see what I could see on the scope, this after putting an isolator plug under the scope, and z took off and ran about 2" all by itself. There is not currently a connection from house static except that which it gets from the 254 volt hubble socket. Hooking that up seems to make it worse, confirming the ground loop theory. The filter I have between the drop cord temporarily powering the motor drives, has its noise capacitor common point on a standoff about 3.5 inches from a huge ground lug, and theres half a volt of crap between those 2 points. Most of it at the 17 kilohertz switching rate, some at 19kilohertz from the smaller 42 volt supply. But the switch point rise & fall times are at the bandwidth limit of my v-1065, or something under 5ns, with a trailing ringing at about 35MHz. This is what's playing hell with the SPI bus. I have some samples of ti's capacitative isolator chips coming, and will carv up a pcb when they arrive and see if they help or hinder. Anything to break up what has to be a ground loop. Should I connect the - terminal of the 5 volt supply to the box ground? ISTR trying that a couple times with essentially zero effect. Or should I go box shopping, and put the motor supplies in their own box? Or should I common the - terminal of all 3 supplies? I think. as long as the supply makers are using the pcb mounting sheet metal as a heat sink, I am screwed, and the best thing I can to is put them on isolated mounts in their own well grounded box. Or try the isolators in the SPI bus when they get here. That bus has been looked at, and despite its 30+Megabaud speed, the waveforms are pretty decently square, and its only common mode ground noise I can see dimly in the background. > If yes, then you have a ground-loop. The signal's GND and the PSU's > GND have two ways of interconnecting, through the PSU/common point and > through the signal GND. This is why the encoder cable was isolated from any grounding except at the 7i90. This is why the drivers all have opto isolation on their inputs. > > 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. > > That is a good indication of a ground-loop. You are obviously > receiving a lot through an antenna function in the wiring. Agreed 150% Peter. > > The idea Bertho is to see if there is a bulletproof method that will > > get rid of the noise. > > Ha, bulletproof, that'll be the day. It often turns out that there is > a brainfart in the design or implementation somewhere (and I too > generate plenty of brainfarts). > > However, going as far as saying "there is a bulletproof method" would > be stretching it. You probably have to rethink your assumptions more > than once and go very very small step-by-step in the buildup and > assembly. Each addition can then be seen in the context of the whole > and see if and where it breaks down. Then, at least, you know where to > look. Applying the bulletproof vest may still be hard work. No argument there either. Thanks Peter. Have a better 2017. 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> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
