Hi Even if everything is "perfect" with the injectors there is the drop in the ethernet wires to consider.
POE is normally limited to 400 ma for a very good reason. The wire used in most cables is 24 gauge (US). Each lead is commonly rated at about 1/2 amp maximum in cable bundles. Resistance is 25 ohms per 1,000 feet. That's all for pure copper. Some stuff has a steel core and it's ratings are much lower on current and higher on resistance. I doubt you will find steel core unless you look pretty hard. The USB hub "wall wart" I just grabbed is rated at 3 amps / 5 volts. That's one for a 4 port powered USB hub. At three amps your ground pair would rise 37.5 volts in a 1000 foot run. A 10 foot run would get you above .3 volts. I would not want to go much past .3 volts in order to keep everything "happy". The other side of the problem is that at ten feet the 5 volt supply is now a 4.2 volt supply (at 3 amps). That's probably less than your devices want to see. They may try to pull more current to make up for the drop (constant power load). All of that does not take into account any problems with the connectors. It also assumes that the wire is full size. I'd look for another way to power the hubs ... Bob On Jul 25, 2007, at 3:39 AM, Stanislav Meduna wrote: > Brad Karp wrote: > >> So in full, the picture is: >> >> PoE 12V >> port0 ------------> DWL-P50 ----> net4826 >> / | >> HP PoE switch | USB cable >> \ | >> port1 ------------> DWL-P50 ----> DUB-H7 USB hub >> PoE 5V > > Generally the USB hub expects to be powered either from the > device it is hanging on, or externally where the power > supply is galvanically isolated from the data lines. > Reason: There is no guarantee that a signal ground > of the data lines is the same as the power ground > of the power supply. So either they have to be the same > (internally powered), or isolated so that they can > float to the same potential. > > Secondly, I don't know about PoE, but I don't think that > it guarantees much beyond that the device gets its power. > The signal grounds between the two devices on a PoE might > be the same, or might be 48V apart. > > Depending on the devices used what you are trying to do > might or might not work, but is surely unsupported at best > and can kill the devices used at worst. > > Generally, loops with power involved are bad. Very bad. > Note: ethernet connection itself is galvanically isolated. > >> We find that while the net4826 can talk to the USB hub when the USB >> hub is *not* externally powered (i.e., unplug the 5V cable in the >> above picture), the net4826 fails to talk to the USB hub when the USB >> hub *is* externally powered. > > If the signal and data ground differ on the net4826 or the > ground potentials of the DWL-P50s differ, the hub gets one > idea of ground via the net4286 and the other one via power > supply. Bad. > >> Moreover, we seem to have *permanently* killed the internal USB >> hub on at least one of our net4826 boxes by connecting an externally >> powered USB hub in this way. > > Well possible. > >> Before you tell me that we're powering the hub incorrectly, consider >> that when we power the USB hub in exactly this way over PoE, it works >> flawlessly when connected to *any other* type of desktop PC we've >> tried. > > Depends on the devices used. > >> Even more strangely, when we power the USB hub not using PoE, but >> using the AC adapter supplied with the USB hub, the USB hub works >> *fine* when connected to the net4826. But the AC adapter and PoE >> splitter output are both extremely close to 5V. > > Because the AC adapter is galvanically isolated from the rest > of the world. > >> We're mystified. Has anyone seen behavior anything like this >> elsewhere, or might anyone have a hypothesis as to why PoE behaves so >> strangely with this combination of devices? > > In the presence of the loop in the power supply connections > this behaviour is completely logical. > >> We're trying to find a work around as quickly as possible, as we need >> to purchase dozens more small PCs ASAP. We'd like to use more >> net4826es, but the problem with USB and PoE is a showstopper for us, >> as the main role of these boxes in our lab is to control USB devices. > > You need to galvanically break the power loop. Either power one of the > DWLs via an AC adapter instead of via PoE, or the Soekris, > or the USB hub. > > If this is not possible, there are galvanically separated DC/DC > converters that can help, but I have no idea how much cost > they add to your setup. > > One final note: an externally powered hub can be quite demanding - > 500 mA per port. The net4826 will also draw a good 1A. Are you > sure the DWL-P50 can handle it without melting? > > Regards > -- > Stano > _______________________________________________ > Soekris-tech mailing list > [email protected] > http://lists.soekris.com/mailman/listinfo/soekris-tech > _______________________________________________ Soekris-tech mailing list [email protected] http://lists.soekris.com/mailman/listinfo/soekris-tech
