You've probably noticed many of my ideas are intending to take advantage of POE, sadly the Netdunio and Netduino plus aren't going to support that at the first release (fingers crossed for an Netduino++). So for now I'll be using a
DLink DWL-P50 which I wrote about on my main blog.
For the Arduino fans
Freetronics have recently started selling an
Ethernet shield with POE, Yay for Freetronics! At the time of writing the 802.3af compliant hardware isn't available but that's in the pipeline, and being Arduino based it's open source hardware, so hopefully the Netduino team can use the hard work that the freetronics folks have done and implement POE on the Netduino in future versions. (Fingers crossed!)
You might not have heard about POE, it's a system that delivers electricity (about 50 volts) through the Ethernet cable. It's really common in offices now days where VOIP phones have been installed. If you've got a VOIP phone have a look at the wires going in. Theirs just one, the network cable, the phone gets it's power from their as well.
The first generation of POE (802.3af) can supply 15W of power over a cat5 cable. In terms of power loss through cable and conversion the actual usable amount is more like 9W. A newer high power POE Plus (802.3at) is out which will be interesting for the high power LED project as it can deliver around 30 watts.
Is it expensive? Well yes and no. If you want a gigabit POE 802.3af compliant switch, yes, very expensive (ca. £600 for a gigabit switch eBay). However companies are upgrading their hardware all the time and the original roll out of POE used 10/100 switches, these are getting upgraded to Gigabit POE switches and the old ones are hitting eBay. Guess what, 10/100 POE switches are perfect for the Netduino projects. I managed to pickup two 24 port managed 19” rack mount switches on eBay for £99 each. You can get small switches for around half that price.
Maybe you want to gave a gigabit switch in their as well for hooking up your PC's, media centers, TV's etc, but theirs no need to get a gigabit POE switch (unless you can get it at a good price!). You might want to look at Linksys gigabit switches as they have a PD (“Powered Device”) that you can hook into your POE switch so it gets the power from their and delivers the networking as well.
You can also get midspan's and injectors, these tend not to be 802.3af compliant, but are still good for a number of projects. Just be aware that if you build your project using a non 802.3af injector then when you get a proper POE switch your project wont handshake correctly with it and you will be left with a device with no power.
There are a few things I really like about the idea of using POE to power these devices.
- Reduced power loss
- UPS backup in a single place
- Power control through the switch
- No need for a electrician to install power outlets if none are available
If I power my ADSL router from a mains adaptor it uses something like 13 watts, when I unplug the router from the wall adaptor it still uses 10 watts!, that's 10W lost in the adaptor. By using a POE switch to delivery the power the consumption of the switch goes up by only 3 watts when I power the router from it, saving (or rather not wasting) 10W.
The switch I have uses about 30 watts as a base line, it's a 24 port POE switch so you only need to power 3 devices to negate that, and changes are you would have been powering a non-POE switch anyway, so maybe you'd break even with two devices (ADSL router and a WIFI adapter?).
To get the best out of the ADSL line it's common to place the router by the phone master socked (I gained about 1-2Meg d/l speed by doing that), however the rest of my networking gear is in the loft. I don't really want to have to buy a dedicated UPS just for the router.
Because my POE switch in the loft is already powered through a UPS it means that if the power goes off I don't loose my network, I can live without it, but a small brownout can put the switches into a funny state, and that's happened a lot with my ADSL router. So by having a good UPS on the POE switch and powering the ADSL router from POE I've reduced the need for a second UPS on the ADSL modem and kept it alive during a power outage.
The other nice thing about that setup is because the switch and router are low power they last an age on a cheap UPS!
The Netgear and 3Com POE switches I have both allow you to power manage your POE devices and monitor the power they are consuming. This is a great feature so if I want to shut some devices down I can do, I can also prioritise auto-powerdown of the devices if the switch goes over power (both switches can supply about 200W, that's a lot of Netduino's!, and at 15W max a port we can get about 13 full power devices running of each switch which is also really good.
POE on WikiPediaSingle port 802.3at POE injector (£34.95)4 port POE 802.3af (£53)