EM4022
Protocol combinations
Protocol saturation
The free-running and the two basic bi-directional
protocols, switch-off and slow-down, can all be combined
with the Fast protocol to give six different protocols, i.e.
Normal free-running, slow-down, Normal switch-off, Fast
free-running, slow-down, and Fast switch-off.
The following should be noted about the different
protocols:
As the number of transponders in a reader beam is
increased, the number of collisions increase, and it takes
longer to read all the tags. This process is not linear. To
read twice as many transponders could take more than
twice as long. This effect is called protocol saturation.
The normal free-running protocol saturates the easiest of
all the protocols, because it does not have any means of
reducing the transmitting population. The Fast protocols,
on the other hand, are virtually immune against
saturation, as they prevent collisions by muting all
transponders except the transmitting one.
1) The switch-off protocols must be used for counting
applications.
2) All the protocols except the switch-off protocols have
built in redundancy because of the fact that they can
transmit a code more than once.
One way of delaying the onset of saturation, is to reduce
the initial repeat rate (not data rate) at which
transponders transmit their codes. This is done by
increasing the maximum random delay between
transmissions.
3) Normal free-running is the only unidirectional
protocol. It has the lowest power spectrum requirement
because the reader transmits a CW wave.
4) Fast switch-off and Fast slow-down are the fastest
protocols, and should be used where speed is important,
or where the data rate limits the reading rate. Fast slow-
down is slightly slower, but theoretically has a lower error
rate.
Figure 14 and 15 below show's reading times for some
possible versions
Optimum repeat delay settings
5) For 125 kHz inductive applications using a 4 kbit/s
data rate, Fast slow-down is probably the best overall
protocol.
The following table lists the optimum repeat delay
settings for each of the protocols vx number of
transponders in a group.
6) For RF applications using a 64 kbit/s data rate,
normal free-running protocol is probably the best
protocol.
Protocol
Number of transponders
3
10
30
16k
4k
4k
1k
1k
1k
100
64k
16k
16k
4k
Free-running
Slow-down
1k
4k
Reader determined protocols
1k
1k
If the reader does not send MUTE signals to
transponders that were programmed for one of the FAST
protocols, the protocol merely reverts to the equivalent
normal protocol. Similarly, if the reader does not send
ACK signals to transponders that were programmed for
SLOW-DOWN or SWITCH-OFF, the protocol reverts to
a FREE-RUNNING protocol. In this manner, the reader
can determine the protocol that is used.
Switch-off
1k
1k
Fast Free-running
Fast Slow-down
Fast Switch-off
256
256
256
1k
256
256
1k
1k
Note, however, that unless
a
transponder was
specifically programmed for the free-running protocol, its
GAP input must be pulled down.
This happens
automatically in low frequency inductive applications,
where the GAP input is pulled down by the internal GAP
detector diode. In RF applications, however, the GAP
input will have to be pulled down explicitly.
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