What sort of data do credit card readers output? I know that many have drivers that automatically translate data into something useful, but what sort of raw data is there? I'm referring to card readers like the free ones given out by Square that input data via the microphone port of a device. Do those sort of readers just pull raw data straight from a card to be interpreted by the software, or do the card readers themselves do any sort of data formatting.
If Square are giving away readers why not get hold of one and have a look at it?
I've worked in the credit card industry and seen the transition from magnetic stripe readers (which give you an ASCII read of the data available on the mag stripe) to the chip card readers which need to decrypt the data from the chip.
The technology is not hard to use, in fact the companies selling the readers want to make it was easy as possible for you to read the card data when you have permission to do so.
What is your motivation for wanting to read the data?
If you are developing a commercial app which will read card details to take payments then interpreting the card reader data stream is the least of your worries, it's all the certification you will have to go through which will cause you headaches.
EDIT: Following up on the Mag Stripe link posted by DwB here is a link to background information on Chip & Pin with links to additional resources.