Jesse Hammons wrote: >My intuition is to use USB cabling, since it already supports >transmitting power and data on the same cabling. But I don't want to >have to support the actual USB protocol (to expensive). From what I >can tell, I might be able to use RS-422 to make a differential serial >signal and run that over the USB cabling. You need to watch the impedence of the twisted-pair cabling you're using with differential signalling (and ideally with RS-232 as well, but that seems to work over any old cable in my experience). If USB cabling doesn't have the right impedence then just get a roll of double twisted-pair cable and run data over one pair & gnd+power over the other. Remember to correctly terminate the cable with the appropriate resistors. If you want a bus type network (multiple devices connected to the same cable) then look at RS-485 - this is much the same as RS-422 but the drivers can be put into a high-impedence state for when something else is driving the line. If you're producing wearable stuff then check the power consumption of the drivers, some Maxim RS-485 drivers I used in a body area network (BAN) prototype (which was intended as a protocol demonstrator only and so didn't actually need to be wearable (which was good, since each interface card was 4"x6")) used a whopping 50mA each, which is rather a lot on a microcontroller wearable setup. If you need to be able to do collision detection on the network (obviously only necessary on RS-485 type nets) then an easy way to do it is to take the input going to the line driver and XOR it with the output from the line receiver. With suitable clocking to allow for the propagation delay through the line driver you get a signal which can produce an interrupt on a collision. Since you'll inevitably get power drops and noise on the long power line in your network it's a good idea to have the network voltage level somewhat above the voltage level required by each device and then use a DC-DC convertor in each device, plus a good smoothing capacitor, to get a nice stable power supply. >I would be interested in hearing about how people have solved the >problem of running data and power along the body. Anybody been able to >have a device on your foot send power and data to a device on your >hand? What datarates did you get and what cabling did you use? The first BAN demonstrator I built, back in 1998 for a company internal research show, used unshielded wiring, non-differential 0V/5V signalling, a single +5V supply with no regulators on each device, and had a 9600bps data rate. This worked well over a two-week exhibition in a large room full of unshielded prototype electronics. The demonstrator was built into a 7' tall wooden cutout of a person and so had some rather long wire lengths. Tests with RS-485 drivers showed that they worked up to megabit speeds over a 20-30' length (I think, this was several years ago) of deliberately tangled twisted pair. (We then moved our protocol development work onto networks of PCs linked over ethernet and I stopped needing to do hardward dev. work.) >I suppose wireless would definitely work but that is much more >expensive and much lower data rate. And of course the power line is >still required. Wired connections between wearables are a much overlooked idea. If the wiring can be built into items of clothing then the wired network becomes invisible and you have the huge advantage of not needing batteries in each wearable device, which means that they can be very small. You do then have the problem of linking the devices to the wires (choose your plugs and sockets carefully) and the wires between the various items of clothing together. You'll soon get tired of having to unplug things like jackets before you can remove them so perhaps the best solution would be a wired network with very short range wireless links to bridge the gaps between various items of clothing. Asher. -- Subscription/unsubscription/info requests: send e-mail with subject of "subscribe", "unsubscribe", or "info" toWear-Hard Mailing List Archive (searchable): http://wearables.blu.org Please, *PLEASE* don't subscribe through a forward/expander/false domain
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