Digital Signal Processing Reference
In-Depth Information
Table 7.6
General rule for forming the address parameter in a binary search tree.
In each case, bit (X) is the highest value bit of the received transponder address in
which a collision occurred in the previous iteration
Search command
1st iteration range
n
th iteration range
=
REQUEST
≥
Range
0
Bit(X)
=
1
,
Bit
(
0toX
−
1
)
=
0
REQUEST
≤
Range
SNRmax
Bit(X)
=
0
,
Bit
(
0toX
−
1
)
=
1
Start
=
1st iteration
1
0
0
0
1
1
1
1
0
1
0
1
0
0
1
0
0
1
0
1
0
1
0
1
0
1
1
0
1
0
1
0
1
0
1111
0
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
=
Transponder from
example
2nd iteration
3rd iteration
Figure 7.22
Binary search tree. An individual transponder can finally be selected by a succes-
sive reduction of the range
Table 7.7
Possible serial numbers in the search range after the
evaluation of the 2nd iteration. The transponders marked (
∗
)are
actually present
Bit number:
7 6 5
4
3 2 1
0
Received data at reader
101
X
001
X
Possible serial number A
101
0
001
0
Possible serial number B*
101
0
001
1
Possible serial number C*
101
1
001
0
Possible serial number D*
101
1
001
1
The renewed appearance of collisions in the second iteration necessitates a further
restriction of the range in a third iteration. The use of the rule in Table 7.6 leads
us to the search range
≤
10101111. The reader now transmits to the transponders the
command REQUEST (
≤
10101111). This condition is now only fulfilled by transponder
2 (10100011), which now responds to the command alone. We have thus detected a
valid serial number — a further iteration is not necessary.
By means of a subsequent SELECT command, transponder 2 is selected using the
detected transponder address and can now be read or written by the reader without
interference from other transponders. All other transponders are silent as only a selected
transponder responds to a write/read command — READ DATA.
After the completion of the write/read operations, transponder 2 can be fully deacti-
vated by an UNSELECT command, so that it no longer responds to the next REQUEST
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