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Low Power Ethernet Hubs?
mike.851 asks: "I'm an engineer working in the embedded systems field. For the past few years, I've been using RS-232 to interconnect various modules into the embedded systems that I build. Lately, I've found that RS-232 simply isn't flexible enough for my growing needs. I need to switch to something else, and I don't think that USB is really an option for me. I've been looking into embedded ethernet as an alternative. I've found several great low-power platforms for hardware development including several of the uClinux kits as well as products from Rabbitsemiconductor. However, I'm having trouble finding ethernet hubs that meet my power requirements (my goal is 2W or better for the hub). So far I've found Ctrlink and W-linx, and I've considered replacing the power supplies in commodity hubs with switching regulators. Does anyone have experience with these products? Does anyone know of other low-power hubs? Thanks."
You could always take a step back in time and use 10base-2. Yeah, coaxial cable sucks and you're limited to 10 Mbps.
Also, I have no idea if any embedded ethernet controllers support BNC connectors, but this would allow to you take one device out of the power consumption equation.
Starting references: here and here.
I ran into a similar perdicament last year where I needed a low power ethernet switch/hub to connect two wireless units on a tower where the cable lengths to the ground were too long.
We ended up using industrial ethernet switches from Sixnet. The price wasn't _too_ bad, I seem to recall $500 for a 5 port switch. Their site is: here
Taking a look at the specifications located here they have units in the 2W Range.
I hope this helps. BTW, we've had no problems with the unit, even after a lightning strike that nuked two of the connected radios.
While Ethernet is one option, it might be a poor fit for your needs.
Some of the things you left out were the ability to survive in harsh environments, dirty power tolerance, price considerations, distance, etc.
There are a number of other bus systems that have made a few trade offs in these areas vs. total speed. Many of them are derived from work for the aerospace and automotive markets.
A few systems you might consider are:
LIN - very very low cost, but probably not fast enough for you.
CAN - up to 1 mb/Sec, lots of low cost off the shelf parts available, developed mostly by Bosch.
Flexray - faster yet, parts are more expensive, but it is faster
MOST - very fast, but targeted at moving multi-media data.
TTCAN - a modification of CAN for real time.
Most of these will support power on connects/disconnects, and are very fault tolerent.
Places to go:
www.flexray.com
www.can.bosch.com
www.ami-c.org/downloads1.htm
These are automotive focused (I'm NOT a "Car guy"), but that is because the industry is very cost aware and very big into reliability. What they tend to use is 10 year old technology that has been beat to death, has all the bugs worked out, has bunches of development tools, and all the hard parts (chips, in this case) are commodity items.
----- Lotus Super 7 - A real car.
Stay away from 100Mb ethernet- by design, if you have a 100MB link, there is a continuous stream of a pseudo-random code being transmitted by each side, which means, basically, 40mW per port, no matter the activity. 10Mb uses much less frequent link test pulses. You can also go in and remove the LEDs, which are probably unnecessary in your application. If you really want to go hard-core, you can hack your way into the part using the MDIO interface (Similar to I2C) and disable unused ports individually.
The Altima AC205 is an "ultra low power" part that may be used in the hub you want. It is 100Mb, but to use the power hungry 100Mb mode, both sides have to be 100Mb.