Am 02.06.2016 um 23:10 schrieb Loren Amelang: > > it seems to me the cable capacitance is entirely symmetric. > That's a capacitance which cancels out both signal and noise (of the same frequency) the same amount, so it doesn't help anything in improving signal/noise ratio.
When your signal has lower frequency than the noise (so the capacitors would have a higher resistance for signal than noise), the receiver could tell them apart anyways. It has a low pass on input. > > But it seems to me like you've drawn cable capacitance from the two > active conductors to some separate ground > That's earth. You have a cable capacitance against earth. Unless you put them on a pole, with no shielding and such. Yes, because those capacitances against earth are the ones which matter for noise cancellation because of using twisted pair cable. If you used two symetric driven signal wires, the whole circuit would be symetrical, and you had full noise cancellation. But Onewire isn't symetrical as soon the ground wire is connected to earth with low impedance. As usual for PCs. For a battery-powered single-bus circuit, ground isn't connected to earth, so it doesn't matter. But it matters again as soon you have two branches which share the ground line. > (All my > documentation sources say unused conductors should not be grounded.) > That is because grounded conductors add additional capacitance to earth. > >> Mobile phone is the most likely culprit. > > Only one carrier has service here, and they're ~8 miles away, under > -90 dBm. And they're CDMA, typically invisible even to intentional > "bug detectors". My phone can be anywhere in the house, makes no > difference. But there is a US "Military Operations Area" over the > next ridge... > Yeah, well... radar then. > > I just searched my collection of 1-wire documentation. All of these > specifically recommend twisted pair: > > Maxim 132 1-wire requirements Quick Guide.pdf Maxim 148 1-wire > guidelines.pdf Maxim 244 Advanced 1-Wire Network Driver.pdf Maxim tb1 > 1-wire hdwre interface.pdf > > The last one, "Tech Brief 1", has a deep discussion of cable > parameters, with great graphs of the relevant effects. "Figure 6 > Electrical Equivalent Circuit of the 1-Wire net" is way more complete > than my drawing. > > This document also recommends twisted pair, and provides many > interesting clues: > <http://www.108relays.ca/dl/1_Wire_Design_Guide_v1.0.pdf> > > "... examine the twist count for each pair and use the pair that has > the most twists per inch." > Sorry, but twisted pair doesn't help you when one of your conductors is connected to earth at some place. And even if not, things get extremly complicated if you have more than one branch which misuse the common ground as a "symetrical" signal wire. > They show that the "Silver Satin" flat phone cable previously used > for 1-wire is actually worse for capacitance than CAT5. "Note that > the capacitance figures are lower in the twisted pair cable, both in > the wires [pairs] and in between wires. This is very important, as > wire lengths get longer." > But that's a property of the cable used and not a property of twisted pair. Ultra-cheap flat telephone cable in addition has a much smaller cross-section than even cheap Cat5 cable. The added resistance makes the cable a low-pass and that's bad for Onewire communication. > > How's this for detail - it matters exactly where on a daisy cable you > attach your slaves! "FIGURE 3 - Any discontinuity on the line, > including slaves can cause reflections. Locations that are at integer > fractions of the line length are particularly problematic as they can > resonant." > Yes. Kind regards Jan ------------------------------------------------------------------------------ What NetFlow Analyzer can do for you? Monitors network bandwidth and traffic patterns at an interface-level. Reveals which users, apps, and protocols are consuming the most bandwidth. Provides multi-vendor support for NetFlow, J-Flow, sFlow and other flows. Make informed decisions using capacity planning reports. https://ad.doubleclick.net/ddm/clk/305295220;132659582;e _______________________________________________ Owfs-developers mailing list Owfs-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/owfs-developers
