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Al Williams

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Baader-Meinhof and Powering Embedded Systems

October 19, 2012

Have you ever heard of the Baader-Meinhof effect? Well, if you haven't, you will soon, and no doubt many times. That's because the Baader-Meinhof effect is how you become aware of something and then suddenly you see it everywhere. For example, you might see an ad for a particular kind of car you've never noticed before and then suddenly they are everywhere. Or upon learning a new word, you might hear it many times in the space of a few weeks. According to Wikipedia this is better known as "frequency illusion" (according to this article the term was coined by Terry Mullen in a letter). Wikipedia defines it as:

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The illusion in which a word, a name or other thing that has recently come to one's attention suddenly appears "everywhere" with improbable frequency

I have often found that I suffer from Baader-Meinhof learning. That is, I will learn something that I have apparently no use for, and then in a matter of days — sometimes hours — someone will come up to me and ask me about that very thing. I've gotten used to it and it is great for your reputation, so I'm not complaining.

So it didn't surprise me when I had been reading about Power over Ethernet (PoE) and two days later someone sauntered into my office asking about it. Granted, I had been reading about a vendor-specific extension to the standards but it did keep me from saying "Uh…. What?" when asked how we were going to run power to an awkwardly placed piece of equipment.

Now that so many embedded systems use Ethernet connections, it makes sense to consider carrying power over the same cabling. The simplest method is to recognize that a standard 10/100 Ethernet port has two unused pairs and just hijack them for low voltage DC. There are quite a few "legacy" devices that do just this and you can find cheap adapters that are little more than power jacks and Ethernet jacks.

However, the lack of a standard means you have to be careful. Cables have to wire correctly with respect to polarity and also voltage levels. Ethernet cable isn't particularly good at carrying power, so expect to drop a bit of voltage over a long cable, too. Worse, not all Ethernet cable has the typically unused pairs and gigabit Ethernet uses all the pairs so you can't just steal a few free pairs.

The IEEE 802.3af-2003 standard addresses most of these issues. It defines a way for PSE (power sourcing equipment) to provide 15.4W on an Ethernet port to a powered device (PD). The PD has to accept some loss in the cable, so the standard only assures 12.95W at the PD end (and don't forget to account for the efficiency of whatever DC to DC converter you will use to get that down to the real voltage you need). Usually the voltage at the source is 48V although 44V to 57V is within standard (and the PD should expect voltage as low as 37V). The power can be on spare wires but can also use "phantom power" to inject the DC into the data pairs.

A newer standard, IEEE 802.3at-2009 (or PoE+) offers 22.5W and uses two wire pairs. However, some vendors are offering equipment that uses all four pairs and claims to work at 51W.

With a standard, you should be just plug and play, right? Hardly, and that shouldn't surprise you. PSE devices can support mode A or mode B (or both). In addition, mode A can have three different configurations.

Mode A uses the data pairs that are shared with 10/100 connections. In addition, both pieces of gear have to agree on polarity (MDI or MDI-X). A lot of devices now autosense (auto-MDIX), so that's one less thing to worry about, at least most of the time.

Mode B uses the spare pairs to carry power which, of course, assumes you have spare pairs. A standards-compliant PD has to accept either mode. The PD provides an electrical indication on the cable that it is able to accept power (the indication is a passive resistor). Without this resistor, the PSE will not send power to the device, so it should be safe to plug in non-PoE equipment to a PSE. The standard allows negotiation to request more power from the PSE.

How do you use this practically? The simple answer is to buy compatible things and connect them. The main thing you have to worry about is how much power the PSE has to provide (and consume, if you are power constrained). You also need to understand that the cables you are using will support the mode that the PSE uses.

Of course, that assumes you are buying off-the-shelf gear. Most of the time, at least part of my system is something I am building, and not buying. Worse, sometimes there is older legacy equipment that I don't have the flexibility or funding to change.

There are plenty of options to refit existing equipment or add PoE to your own designs. Of course, in a completely proprietary set up, if you have the extra wires you can just use one of the cheap "injectors" and "splitters" to add and extract power to the unused pairs, assuming you have them. You can control the voltage and polarity and you only have to worry if you think someone might try to plug something you didn't expect into the system. Be sure to think about a technician in the future using a cross cable, too.

If you want to stick with the standards, you can get compliant injectors (known as "midspans", although most of them I have found are purely mode B. Nearly all the mode A devices I've seen are "switches" (that is, an Ethernet switch, usually with one regular port and a number of PoE ports). Even then, some switches use mode B, also. Using a midspan and a splitter, you can put power in on one side of the cable and take it off at the other side while the conventional Ethernet gear attached is ignorant of the whole thing.

The other option is to build PoE into your design. There are chips from vendors, of course, to help. However, if you aren't designing a volume product you are probably better off with a module. Silvertel has a range of modules that do most of the work for you. Depending on the version you pick, you might use a programming resistor or two to set parameters and the module delivers regulated output in the voltage you select.

They aren't the only vendor of such products, of course. For example, see Moatel and Powered Ethernet. If you want to roll your own, there are chips available from the usual suspects, including Linear Technology, Maxim, National, TI, and probably plenty of others.

I don't know about you, but I'm holding out for Tesla's wireless power transmission to be reinvented. Maybe that will lead to powered Wi-Fi! Meanwhile, I'll have to settle for carrying power over the network cables.

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