My goal, here, is to talk about the low level fundamental communications that seperate the evil analogue underworld from the lovely, friendly digital fairy garden, where we all like to play.
And it all begins with our friend "induction".
At the very low level, RFID/NFC relies on the fact that if you energise a coil and place another coil near it, the second coil will pick up some of that energy through induction.
Moreover, the two coils become magically (or magnetically, depending which world you come from) \'coupled\',
so it\'s possible for the second coil to effect the voltage on the first, and it does this by shorting itself out.
If it does so, there will be a drop in the voltage on the first coil, and this is called "DAMPING".
That\'s it. In a nutshell, that\'s how RFID works:
the coils \'talk\' to each-other by either sending energy (from the READER), or causing an energy drop (from the TAG).
In a little more detail, what happens is this (and for the purpose of this section, we\'ll assume the TAG is the dumbest type that just spits out an ID):
the READER energises its coil by powering it on and off repeatedly.
For a standard LF system, this will be 125,000 times per second, or 125KHz.
This is known as the \'CARRIER\'.
The TAG, when placed in this field, will scavenge some power from its now inductively-coupled coil and come to life.
If the reader needs to send an extra \'wake up\' (or any other) command, it can do so by simply switching it\'s CARRIER off altogether for short periods.
The TAG stores enough energy that it can keep running for long enough to interpret these gap periods, even though it\'s temporarily lost its power source.
The length of the CARRIER signal between the gaps will usually signify a 0 or a 1, so like this the READER can send binary messages.
In other words, they\'re talking \'ASK\': Amplitude Shift Keying.
Data is sent by shifting the amplitude of the signal.
More accurately, they\'re talking \'OOK\': On Off Keying.
A message going from the READER to the TAG is signalled by the CARRIER being ON or OFF
and the message coming back is either DAMPED or UN-DAMPED.
Things get a lot easier to understand if we visualise them, so here is a plain 125KHz CARRIER viewed from an oscilloscope: