R
EM4223
5. FORWARD LINK ENCODING - READER TO
TRANSPONDER
Commands and data are received from the Reader,
encoded by means of Pulse Interval Encoding. The
Reader transmits pulses in the form of dips in its carrier
wave. The intervals between dips convey information in
accordance with the following description.
One Tari, while the 2nd period of the SOF symbol is equal
to 3 Tari. The first part of the SOF symbol is provided to
allow detector circuitry to settle (should this be
necessary). The second part of the SOF symbol is used
as a Calibration period. The received SOF symbol is
used to calibrate the command decoder every time a
command is received. This calibration is used to
establish a pivot to distinguish between subsequent data
‘0’ and data ‘1’ symbols. The pivot point has a value of
1.5Tari and is derived from the 3Tari interval contained in
the 2nd part of the SOF symbol. The binary data ‘0’ and
‘1’ are extracted from the incoming data stream by
comparing the inter-pulse interval with a pivot point. If the
interval is less than the pivot, then the binary value is ‘0’
and if it is greater than the pivot then the binary value is
‘1’ (See clause 0). Each time that a new command is
received by the Transponder, the SOF re-calibrates the
decode counter thereby compensating for any variation
in the Transponder clock frequency due to changes in
RF excitation levels or temperature variations. The circuit
has been designed to accommodate a Transponder
clock frequency variation of ±40% from nominal. The
basic Tari period as transmitted by the Reader is
specified in Table 16 and illustrated in Fig. 9.
The Transponder responds to transmissions by the
Reader as described herein.
Carrier modulation pulses
The data transmission from the Reader to the
Transponder is achieved by modulating the carrier
amplitude (ASK). The data coding is performed by
generating pulses at variable time intervals. The duration
of the interval between two successive pulses carries the
data coding information. This is known as Pulse Interval
Encoding, (PIE). The Transponder measures the inter-
pulse time on the high to low transitions (falling) edges of
the pulse as shown in Fig. 9
Basic time interval – definition of “Tari”
The time “Tari” specifies the period in microseconds
between two falling edges representing the symbol “0”.
The word “Tari” is an acronym for “Type A Reference
Interval Time” as defined in the ISO18000-6 Type A
specification. The period between the two falling edges
defining each of the other symbols is based on a multiple
of the basic Tari period. The SOF symbol (Start of
Frame) consists of 2 periods, the 1st of which is equal to
Tari
Tolerance
20 µs
±100 ppm
Table 16 - Reference interval timing
Tari
100%
M
Fig. 9 - Inter-pulse mechanism
15
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