I am interested in learning how TAP (e.g. Presto) cards work. It's funny how -

There is no bar-code, no magstripes (black stripe) and no chip visible on the card, yet it has the same functions as a credit card.

You reload money on it online and it stores its own information.

I can't find any information on ISO/IEC 7811 on how it is stored.

I want to ask this question on Stack Overflow:

How is the information stored? How is the card scanned?

  • 2
    Nope, not suitable for Stack Overflow as it isn't a software question. The electrical engineering site may be suitable, but check their FAQ first.
    – Oded
    Sep 4, 2013 at 21:00
  • 1
    I need to set a bounty... Sep 9, 2013 at 17:22

1 Answer 1


I'm afraid your question may not be on topic for any currently existing Stack Exchange site.

Out of the available choices, Oded's suggestion of Electrical Engineering does seem closest, but I don't really see your question falling under any of the topics listed in their FAQ:

  • a specific electronics design problem
  • the theory and simulation of electromagnetic forces
  • a communication scheme
  • the writing of firmware for bare-metal or RTOS applications

That said, they do have a somewhat similar question with seven upvotes, so maybe yours might be OK too.

That said, it's pretty easy to find the basic answer to your question just by Googling for it. The first result I get is the Wikipedia article, which says (emphasis mine):

"The Presto card, which is the size of a gift or debit card, will use “stored-value” in the form of a common electronic purse (e-Purse) as the medium for all fare payment transactions in addition to cash. Electronic readers will scan the customer’s Presto card (which contains a passive RFID chip within the card) as they board their public transit vehicle or enter a station calculating the correct fare and deducting it from their card balance - all within one-third of a second."

(BTW, that text looks like it's been copied straight out of advertising copy for the system. It seems to have been introduced in this edit back in 2009 by a user named "PRESTO card". Still, it's nowhere near the worst case of self-promotion I've seen on Wikipedia, and at least it does provide useful information.)

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