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    The Intel Galileo had serial over headphone jack.

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      Was always my personal favorite router mod:

      The TTL-232R-3V3-AJ cables were cheap to come by too.

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      I was always impressed that the 2nd generation iPod Shuffle used its headphone jack for both data transfer and charging. I wonder if that would be viable for charging any device that actually had to power a display.

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        At 100mah, I doubt it, that battery is tiny in its size.

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        Don’t square et al use the headphone jack for data via analogue audio? I assume it is something like a simple modem, looking for peaks and troughs?

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          Yep, they had a tiny little MSP430 with only a few hundred bytes of RAM decoding the magstripe data, encrypting it, and sending it to the phone. I forget what sort of encoding they used, but I don’t think it was a standard ASK or PSK thing because the MSP430 didn’t have a DAC as I recall.

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          Some keyboards (ergodox, mine) use a TRRS connector for power and I2C

          It’s really not a great engineering idea. You run into issues with shorts (if you plug in the cable while it’s already powered up) and incorrectly manufactured cables. The upside is that the connectors are cheap and small and the cables are ubiquitous.

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            Same thing with the Diverge.

            Seems like it’s practically convention for split keyboards at this point :D

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            The Canary uses it too but it doesn’t seem to work very reliably.