You heard it here first! If somebody makes this and I don't get at least credit for the idea, I'll get lawsuit-happy!
So, here's the problem. I have a PowerTap SL 2.4, a damn fine piece of bicycle telemetry instrumentation. I also have a Garmin Edge 705, which ought to be quite a nice device, except for the fact that it can't read power data - at all - from my PowerTap!
In theory, this should not be a problem. Both of these devices communicate on the 2.4 GHz band and, with some cleverness and payment of royalty fees, I'm sure Garmin and Saris could happily negotiate either a software fix to the Edge 705 head unit, or a hardware change to the PowerTap itself. Rumor has it that this will happen this fall. I'm never one to believe rumor.
So, here's my idea. It's kludgy, and will take some work to implement, but I don't see why it's not possible. I'd like to build a wireless repeater device. Basically, this would function as a PowerTap head does, listening for the data from the PowerTap in real time. It would then masquerade as an ANT+Sport-compliant power device, and relay this information to the Edge 705! The device itself would be the size of a matchbook, run off a CR2032 battery, and probably have a little USB connector for programming the serial number of the PowerTap, or at the very least, a button and a red/green LED. It's a simple repeator-translator, nothing more, nothing less. It's a hack, partially technological, mostly political.
I was turning this idea over and over in my head, and thought to myself: is there any reason why this has to be restricted to the PowerTap SL 2.4 head? Why can't the regular ol' PowerTap SL do the same exact thing? Unlike the 2.4, which communicates directly with the head unit with a range of about three meters, the plain ol' SL communicates wirelessly with a smaller pickup, within a range of three inches, which then communicates via wire. Why not make a device that reads the SL data in real time, and also masquerades as a ANT+Sport power meter, for the benefit of the Edge 705? Not only that, but this device could also speak PowerTap, and basically upgrade a PowerTap SL to a PowerTap SL 2.4! You could even put a little magnetic pickup on the device, for regular old speed, and for kicks, give it the same shark-fin shape.
I tend to rant. I hope my proposal is clear. If not, perhaps ASCII art will work:
PowerTap SL 2.4 --(proprietary)--> PowerTap 2.4 head: this exists.
ANT+Sport device --(ANT+Sport)--> Edge 705: also exists - see the Quarq offering.
PowerTap SL 2.4 --(proprietary)--> Device --(ANT+Sport)--> Edge 705: my idea.
PowerTap SL --(short-range)--> Device --(ANT+Sport)--> Edge 705: also my idea.
PowerTap SL --(short range)--> Device --(proprietary)--> PowerTap 2.4 head: also my idea.
A single device can do all of this.
"Proprietary" - describes the 2.4 GHz protocol a PowerTap SL 2.4 uses to communicate with a head unit.
"short-range" - describes another proprietary protocol, of unknown frequency or characteristics, that a PowerTap SL uses to communicate with a wireless receiver, about three inches from the hub.
"ANT+Sport" - describes Garmin's wireless protocol.
I don't have any time to work on this in May, and I'll be on the road all summer. Perhaps I can work on this when I get back to MIT? The hardware should be trivial, I just need to be able to send/receive a 2.4 GHz signal. Any takers? Garmin or Saris, are you listening?