The headphone jack

[TooNice]

A few years ago, I came across a banking card reader that attaches to an iPad. At the time, I didn't think much of it, expecting that it was a proprietary accessory / feature of the iPads headphone jack.

As the company switched to an Android tablet for taking payment, I was going to investigate something similar for Android tablets, expecting that it would be hooked via microUSB.. but it turned out that the headphone jack reader work just fine.

I never really thought how those headphone jack worked. I mean, they've been around since before PCs were even common, and unlike USB ports I don't think that they were updated since the one I had in my first walkman.. but now I have a passing curiosity.

How is data transfered over? Transfer rate? Any more clever use of the port that we don't see too often?

[DDY]

I'd put my money on it being a software MODEM.

Pretty much the same technology as dial-up.

Basically turning digital data (e.g. card reader stuff) in to analogue audio data, sending it over the audio jack and turning it back in to digital data, certainly using software on the host side (e.g. the tablet).

[Ttaskmaster]

https://techcrunch.com/2016/09/07/applejack/