Step 1How does RFID work?

RFID, or Radio Frequency IDentification is the term used to describe a wide variety of  standards that allow data stored within electronic 'tags' to be read by a reader without using wires.  There are a number of standards, encoding formats, and frequencies in common use.   I will describe the 125 kHz standard that is common for access control mechanisms.

125 kHz RFID tags are commonly encased in a business card sized piece of plastic, or a round disk.  The tag consists of a coil of wire, connected to a microchip.  When the tag is brought into close proximity to a reader, energy is coupled inductively from the reader to the microchip within the tag. 

The energy from the reader has dual use; firstly, it provides power to run the card, and secondly, it provides a communication medium for data to be transmitted.  Once powered up, the tag modulates the bit pattern that is programmed into the tag using a signal that the reader can detect.  The reader then reads this bit pattern, and passes it onto the door controller.  If the bit pattern matches one that is authorised, the door will be unlocked.  If the bit pattern does not match an authorised one, then the door won't unlock.

In the RFID system I was playing with, the bit pattern looked like this;

1111111110010111000000000000001111100010111110111101001111010000

I will describe what this pattern actually means in the next page.

One interesting feature of the data transfer between the card and the reader, is that data is encoded using Manchester Encoding, which is a way of encoding data so that it can be transmitted over a single wire ensuring that the clock information is able to be recovered easily.  With Manchester encoding, there is always a transition in the middle of a bit.  If you want to transmit a 1, the transition would be from low to high, and if you want to transmit a 0, the transition would from from high to low.  Because the transitions are in the middle of each bit, you can ensure that you have locked onto valid data.  For a detailed description, have a look a this page.

The actual data is transmitted by the card effectively shorting the coil out - this applies an additional load to the transmitter in the reader, which can be detected.