Gene is right. Many people don't understand grounds. The star ground is not magic. It is based on two basic laws of physics. Just two things to remember
1) It is a fundamental law of electricity the current ONLY flows if there is a loop. Take a straight piece of wire and connect one end to a battery and leave the other end free and absolutely zero current flows and the wires has EXACTLY the same voltage all along it's length. THIS is why you use a star ground. No current will flow in the ground wire if ONLY one end of each wire is connected to your ground reference. Make a loop and you have current. 2) Mr. Ohm has a law that says if there is current you have voltage. In fact the volts are proportional to the current. So if you make a loop with your ground wires there will be current and hence different parts of the loop will have different voltages on them. It will not be zero everywhere as you might think. Basically the combination of Kichoff's and Ohm's laws. People try can get around this by using real heavy and thick ground wires. But they forget (or never knew) that impedance has an imaginary component and the thick wire only effects the real component. This noise that is transmitted is high frequency AC not DC. you have to make the path "low impedance" if you want to get around Ohm's law. This noise is basically in the radio spectrum, big wires don't help much there. Just make sure ALL you grounds are "dead ends" There are places where where loops sneak in. One of then is shielded cables where the ground shield is connected to each end of the cable. There are more sophisticated systems where they shunt the AC to ground using capacitive coupling but don't even try unless you are a specialist engineer. You do see this in some equipment. They use a capacitor in parallel with a high value resister and they connect signal and protective grounds this way. What they are doing is making a low impedance path for AC to ground. The other thing you can and should do is try to stop this electrical noise at the course. The best way to is again remember Kichoff. Current flows in a loop. So any time to have a wire with current, say going to a motor, some place there is another wire with that same current flowing in the opposite direction. Always route those to wires as close as possible Best way to do that is to twist them together. The opposing directs will cancel any electric field. At about 8 wire diameters, from a twisted pair the field will be nearly zero. So if you d this the noise will only radiate for less then about an inch. Wire routing and layout matters. On Sun, Apr 22, 2018 at 8:10 AM, Gene Heskett <[email protected]> wrote: > On Saturday 21 April 2018 23:50:09 jeremy youngs wrote: > > > So, got the 7i90 today, been swimming through the manuals. > > I haven't yet connected the board , that will likely happen mid to > > late week as I seem to have all of the automotive work i want right > > now ( life could be worse) . I downloaded the support software and > > manuals. I am speculating that the softdmc libraries are preloaded in > > the backup fpga directory on the card ? Am I correct in presuming this > > ? > > ISTR they come unprogrammed, and set to be connected to a parport for > programming the correct config for your use into it with mesaflash. It > has a backup recovery that must be used long enough to reset it to > parport communications IF its been set to talk SPI, which is faster on > the pi's but not available on *86 machines. Except for the missing spi > capability on x86 machines, the parport on the x86 motherboard s/b fast > enough. > > But note, and I emphasize this strongly, the 50 pin connectors hook > directly into the fpga, and noises above 4 volts may blow the fpga's > buffers, so the surge and noise protection of a 7i42TA, which also gives > you a handy mechanical terminal arrangement to connect it to the outside > world, and will protect it from the noise pickup from the stepper motors > etc. I destroyed several before I understood the importance of that, and > the one driving my lathe has had functions moved to different card > outputs because of blown buffers. I actually wound up putting the pi, > the 7i90, and the 3 7i42TA's in a separate box, and mounted that box to > the lid of the old rusty box the power stuffs was in. Then my noise > problems disappeared. > > But this brings up a second recommendation, which is to establish a > single bolt as a common ground point, with the commons of everything > else connected ONLY to this bolt. This is known as a star ground system. > Stuff connected to the nearest ground often results in having more than > one ground and that constitutes a ground loop, which acts as an antenna, > picking up noise from anything radiating it, and this noise can easily > blow gates in the fpga on the 7i90. That bolt should also be the only > place the 3rd rounded in the US pin in the power cord is connected, cut > them off till you've only one left to get rid of ground loops, just > don't cut the last one! > > To give some credence to what I'm writing, I have never been a working > machinist altho I've had my hands of the cranks of a lathe many times in > my 83 years, but I am a Certified Electronics Technician and have used > that knowledge to earn a liveing since I was about 14 years old. I got > interested in broadcasting, and spent the last 22 years of my working > time with an office door plaque saying Chief Engineer on it at some tv > station. And I have the instruments to measure, and visually show me > that noise. > > One thing I did when configuring this lathe, was that since the firmware > you use mesaflash to install puts _most_ of the "canned" functions on > the first of those 50 pin connectors, 24 i/o's per connector, when I > started adding the gingerbread that needed gpio pins's, I started at the > top of the 3rd connector, and I've added quite a bit of stuff, and > figured I'd stop when I had used what was in the middle. That way I > wasn't moving stuff around once it was set, and thats worked out well. I > still have plenty of gpio's left yet, to hook up coolants, lubrication > squirts etc that I haven't bought the hardware to do it with, yet. > > So if you have noise problems, you will need a scope fast enough to see > the noise, and that means 100 mhz of bandwidth, not one of these $40 > toys. Be on the lookout for used Hitachi v-1065's on ebay. Now 35 years > old, somewhat computerized so its calibration has stayed valid, much > moreso than tektronix stuff, its a decent tool yet. Mine has spent many > hours in a twin piston pounder airplane as I've also played visiting > fireman at other broadcast facilities, so I've had to open it and > retighten all the framing screws that vibrated loose, and the > pushbuttons are getting flaky, but the tube is still fairly bright and I > can believe what it tells me. Dual trace, fully triggered of course. > > -- > 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 > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California ------------------------------------------------------------------------------ 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
