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protocol is a common format used in transponders in applications
requiring animal identification, but
is also often used in non animal related industrial identification. It is fully described in the ISO 11784 & 11785
standards publicly available. The description of ISO 11784 & 11785 discuss two forms of transponder
encoding, and these are FDX-B (full duplex), and HDX (half duplex). Here we will discuss the HDX protocol
description. For a description of FDX-B protocol see FDX-B Animal identification protocol description.
Difference between FDX and HDX transponders.HDX is an abbreviation for Half Duplex, while FDX transponders are Full Duplex. A FDX transponder will
transmit its information while the RFID reader is transmitting a magnetic field from its antenna. This allows
FDX tranponders to be continously powered while in the presence of the field. The short comings of this
method is that the large magnetic field transmitted from the the RFID reader will make reading the data from
FDX transponders more difficult as the signal received is relatively small in comparison to the activating
magnetic field. HDX transponders will differ from FDX ones in that they will wait for the magnetic field from
the RFID reader to be turned off before they transmit their information. This means that the reader will detect
the transponder at a greater distance as the signal is not being superimposed on the much larger RF field
from the RFID reader.
HDX frequency and modulation.HDX protocol based transponders are defined to operate in the 134.2kHz band, and employ a Frequency
Shift Keying (FSK) method to transmit their data. To transmit a logic 1 a HDX transponder will issue 16 cycles
at 124kHz, and to transmit a logic 0 it will issue 16 cycles at 134kHz.
The activation field being created by the RFID reader is typically 50mS in duration. During this time the RFID
reader will look for the presence of any FDX protocol transponders and if none are detected the activation
field is turned off to allow any HDX transponders to respond. The RFID reader will wait a maximum of 3mS
for the HDX transponder to start its transmission. If nothing is detected within this time the RFID reader will
again turn on an activation field and repeat this cycle. This is illustrated in Figure 1 below.
Figure 1. HDX protocol example.
HDX data structure.HDX protocol based transponders carry 112 bits of data. The structure of this data consists of:
When the Tag enters the electromagnetic field transmitted by the RFID reader it draws power from the field
and waits for a break in the field before transmitting its data as shown above. The least significant bit (lsb)
is transmitted first. The 8 bit header pattern is transmitted to indicate the beginning of the data block.
This is followed by 38 bits of the identification code. For an animal application this will be the identity code of
the animal. This is a unique 12 digit decimal code for each animal.
This is followed by the 10 bit country code. A country code is a 3 decimal digit value used to refer to
individual manufacturers. A code of 999 is used to indicate that the transponder is a test transponder and
need not contain a unique identification number.
The 1 bit data block status is an indicator flag to indicate whether an additional data block exists. A value of 1
indicates that the transponder contains an additional 24 bit data block. Otherwise it is 0. Following this we
have 14 reserved bits allocated for future use.
The animal application indicator is a single bit indicating that the transponder is used for animal identification.
This value is set to 1 to indicate an animal identification application, and 0 otherwise.
The preceding 64 bit block (excluding control bits) is then used to calculate a 16 bit checksum. The calculation
for this CRC checksum is defined in ISO 11784 & 11785 and is included after the animal status bit.
After the CRC check bits we have the extra data block. This data block exists if the data block status flag is 1.
When the data block status flag is 0 this value will be 000000. The data block may be used to append
additional data relevant to the individual application.
Priority 1 Design carries a stock of low cost rfid reader writer units that are capable of reading and writing
FDX-B and HDX protocol transponders.
We also carry other rfid products for various applications.
RFID Reader Modules
RFID cards & keytags
RFID reader IC's