Modulation Read/Write Device to Transponder: 100 % ASK and Binary Pulse Length Coding
Modulation Transponder to Read/Write Device: Strong ASK modulation with Anticollision, Manchester and Biphase Coding
The following block diagram shows in principle the HITAG system configuration
A control and data processing unit controls the modulation of the carrier signal and processes data coming back from the demodulator circuit.
Absorption modulation is used, when sending data to the read/write device.
To force the absorption of the magnetic field, the transponder in principle turns on/off an internal resistor.
With the resistor turned on, the physical state is named Modulator ON (loaded) otherwise Modulator OFF (unloaded.)
Two different codes are used for the transmission of data to the read/write device:
The first bit of the transmitted data always starts with the Modulator ON (loaded) state.
原文：http://www.cnblogs.com/shangdawei/p/3724575.html