On Fri, Aug 24, 2007 at 03:54:47PM -0400, Bob Keyes wrote: > > We've had a problem here with a NetGear ethernet switch, model FS116P. If you > have the Soekris 4826 connected over PoE and a serial console, you'll blow > out > the PoE port on the NetGear...permanently! Our 8 ports of PoE are now down > to 5. > > Don't buy NetGear!
This message is old, but since it is unanswered and people seem to blindly use PoE without knowing the risks. Here are some details about 802.af technology. PoE supplies a small voltage/current to an unidentified device. After identification it supplies 48V. There is a second voltage range for identifying the power requirements of the device, but I'll leave that out. Interestingly all chips seem to regulate the negative rail, so the +48V is always there, and the GND is not. Normally you would speak of fixed GND and switched -48V. A 802.af device would supply a specific resistor on the power rails to identify. This means with low voltage it will not run. So far all this means that the device will run or not. Now we come to the critical effects. Ethernet is galvanic isolated because it uses transformers. And a normal wallwart supply is galvanic isolated. If a special power supply is not isolated it usually have it's GND grounded. A RS232 connection is not galvanic isolated. If you connect two boxes with RS232 you will electrically connect them, which is usually(*) not a major problem, since they are both ground connected or isolated. (*) There's the risk of earth loops, but that's a different point. PoE however breaks that rule. A normal multiport PoE injector supplies multiple devices from the same source. A simple PoE device will not galvanic isolate to given power, but just put the power into it's normal regulator. Remember: With adding a RS232 cable you connect the ground lines! And the 48V rail is always feeded. The Soekris (looked at 4511 and 4826) implementation is a bit dissapointing for 802.af expectations. I couldn't identify a typical 802.af chip, which means that it doesn't identify itself, but I may have missed that. I can identify a rectifier bridge, which is good. The bridge protects against reversed polarity. It is just a single bridge, which means that it can use only a single cable pair. Someone else already noticed that the Soekris are only using the unused pair. I can't find ay power transformer, which means the Soekris isn't doing a galvanic isolated DC/DC transforming, which it should for 802.af, since it has connectors, which can lead to non isolated connections. You can use the Soekris without connecting RS232/USB (*) to non isolated devices at any supply. (*) That also includes e.h. connecting a grounded WiFi antenna. But if you connect RS232/USB to non isolated devices, you must use an isolated supply. A switch can't be used in that situation, since a switch is never an isolated supply. At best: don't connect the RS232 or USB at all when supplying from a switch. Even a battery powered notebook can only be connected if the notebook isn't connected to e.g. a grounded USB disk at the same time. Don't expect a switch to supply at all, since the Soekris isn't identifying. About the NetGear. It likely have a melting fuse for that situation. Under normal conditions it will start with limited current and will fail identification. Since you connected the ground with your RS232 cable you effectifely disabled the controlling for the switch. I don't know the internal design of the switch, but I can imagine that you basicly shorted the 48V with the RS232. That said: Soekris never claimed their systems to be 802.af conforming. What they have implemented is working perfectly for remote power injection when the device is at a place where you don't have power. You just have to be aware about the limitations and don't blindly connect everything together that has the same kind of plugs. -- B.Walter <[EMAIL PROTECTED]> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm. _______________________________________________ Soekris-tech mailing list [email protected] http://lists.soekris.com/mailman/listinfo/soekris-tech
