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EM4100 Protocol description.

RFID transponders (Tags) are devices carrying digital information that can be read from a distance by a
RFID transceiver (Reader). In order to be able to read the information stored on the RFID tags the reader
must know how the information is stored and the protocol for extracting it. One of the more common data
formats for RFID transponders is the EM4100 protocol, named so because the microchip at the heart of
the Tag is based on the controller chip made by the company  EM Microelectronic.

Reading an EM4100 RFID Transponder.

EM4100 compatible RFID transponders carry 64 bits of Read Only memory. This means that  information 
can be read from the Tag but no data can be changed, or new data  written to the card once the card has
been programmed with the initial data. The format of the data is as shown here.

1 1 1 1 1 1 1 1 1    9 bit  header bits, all 1's
8 bit version  number  D00 D01 D02 D03  P0 
or customer ID. D04 D05 D06 D07  P1 
D08 D09 D10 D11  P2  Each group of 4 bits
D12 D13 D14 D15  P3  is followed by an Even
32 Data Bits D16 D17 D18 D19  P4  parity bit
D20 D21 D22 D23  P5 
D24 D25 D26 D27  P6 
D28 D29 D30 D31  P7 
D32 D33 D34 D35  P8 
D36 D37 D38 D39  P9 
4 column Parity bits PC0 PC1 PC2 PC3 S0    1 stop bit (0)

When the Tag enters the electromagnetic field transmitted by the RFID reader it draws power from the field
and will commence transmitting its data as shown above. The first 9 bits are a logic 1. These bits are used as
a marker sequence to indicate the beginning of the string.  As Even parity is used throughout the data this 9 bit
sequence of 1's will not occur at any other location in the string.  This is followed by 10 groups of 4 data and 1
even parity bits. Finally there are 4 bits of column parity (Even) and a stop bit (0). The Tag then continues to
repeat this string as long as it has power.

Shown here is an example string for a proximity card that has the data   $06 (version number), and $001259E3
as a data string.

1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 0 0 1 1 0 0 1 0 0 0
0 6 0 0 1 2 5 9 E 3

Data modulation.

As discussed in RFID basics  the RFID transponder is able to transmit its data by modulating the RF field of
the Reader. Here we discuss 3 popular modulation schemes.

Manchester Encoding.
BiPhase Encoding.
PSK Encoding.

The Transponder and Reader use the individual cycles of the RF field to syncronize the data transmission
between the two. The frequency of the synchronizing clock then simply becomes the frequency of the RF field
RFID system clock frequencies vary according to the application required.  In low frequency, short distance
sensing of Tags the typical band used is between 100-150Khz. For longer range sensing a system frequency
of 13.56Mhz might be used, or other frequency as the application requires.
Of course the designer of an RFID system is restricted to using particular frequency bands as RFID systems
are radio emitting devices and therefore under the control of the local radio frequency regulator bodies.

The length of each bit is specified in terms of clock cycles. For the EM4100 protocol bit lengths can be either
64, 32, or 16 Clock cycles.

Manchester Encoding Scheme.

With Manchester Encoding a Tag will produce a level transition in the middle of the bit period. A low to high
transition represents a logic 1 state, while a high to low transition represents a logic 0 state.

Manchester Encoding Scheme

BiPhase Encoding Scheme.

Biphase Encoding schemes modulate the RF field so that there is a transition at the beginning of each bit
boundary.  A logic 0 state has a transition in the middle of the bit period, while a logic 1 state has no transition
during the entire bit period.
Biphase Encoding Scheme

PSK Encoding.

With PSK (Phase Shift Keying) encoding the RF field is modulated so that there is a transition with each clock
period. This means there can be up to 64, 32, or 16 transitions per bit depending on what bit length the Tag is
using. When a phase shift occurs it represents a logic 0 state, while a logic 1 state is interpreted when there is
no phase change at the bit boundary.
PSK Encoding Scheme

Priority 1 Design carries a stock of low cost EM4100 compatible Transponders. We also carry a stock of
low cost T5557 RFID Read Write transponders,  You can buy EM4100 and T5557 transponders direct from
our online shopfront.
See T5557 RFID transponder to learn about reading and writing  RFID tags.
    See Specifications.

    RFID keytag 125Khz EM4100 compatible       125khz RFID proximity card  
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