Digital Signal Processing Reference
In-Depth Information
Downlink (reader
=>
REQUEST
<11111111
REQUEST
<10111111
REQUEST
<10101111
1st iteration
2nd iteration
Transponder)
Uplink
101X001X
1X1X001X
Transponder 1
10110010
10100011
10110010
Transponder 2
Transponder 3
10100011
10110011
10110011
Transponder 4
11100011
REQUEST
<10101111
SELECT
10100011
Downlink
3rd iteration
Read/Write
10100011
Uplink
Transponder 1
Transponder 2
10100011
10100011
Transponder 3
Transponder 4
Figure 7.21
The different serial numbers that are sent back from the transponders to the reader
in response to the REQUEST command lead to a collision. By the selective restriction of the
preselected address range in further iterations, a situation can finally be reached in which only
a single transponder responds
Table 7.5
Possible serial numbers after the evaluation of the received data
and taking into account the collisions (X) that have occurred in the first
iteration. Four of the possible transponder addresses (
∗
) actually arise in our
example
it nu ber:
7
6
5
4
3 2 1
0
Received data in the reader
1
X
1
X
001
X
Possible serial number A
1
0
1
0
001
0
Possible serial number B*
1
0
1
0
001
1
Possible serial number C*
1
0
1
1
001
0
Possible serial number D*
1
0
1
1
001
1
Possible serial number E
1
1
1
0
001
0
Possible serial number F*
1
1
1
0
001
1
Possible serial number G
1
1
1
1
001
0
Possible serial number H
1
1
1
1
001
1
The general rule for limiting the search area (range) is shown in Table 7.6.
After the reader has transmitted the command REQUEST (
≤
10111111), all trans-
ponders that fulfil this condition will respond by sending their own serial numbers to
the reader. In our example these are the transponders 1, 2 and 3 (Figure 7.22). There
is now a collision (
X
) at bit 0 and bit 4 of the received serial number. From this we
can conclude that there are still at least two transponders in the search range of the
second iteration. The received bit sequence 101
X
001
X
still permits four options for
the serial numbers that remain to be detected (Table 7.7).
Search WWH ::
Custom Search