P4022 datasheet(6/13 Pages) EMMICRO

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PRELIMINARY

EMMICROELECTRONIC-MARINSA

P4022

with their own protocols automatically after a time

out, or continue immediately upon detection of

an ACK signal indicating that the transmission

which caused the MUTE has been completed.

In the P4022 the MUTE signal is implemented as

a single gap received while the transponder is not

transmitting.

Protocol combinations

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, Normal SLOW-DOWN, Normal

SWITCH-OFF,

Fast

FREE-RUNNING,

SLOW-

DOWN, and Fast SWITCH-OFF.

The following should be noted about the different

protocols:

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.

3) Normal FREE-RUNNING is the only uni-

directional 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.

5) For 125 kHz inductive applications using a 4

kbit/s data rate, Fast SLOW-DOWN is probably

the best overall protocol.

6) For RF applications using a 64 kbit/s data rate,

normal FREE-RUNNING protocol is probably

the best protocol.

Reader determined protocols

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.

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.

This will consume

extra current.

Protocol saturation

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.

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.

Seven different settings

are available from 16 bits to 64 kbits. A higher

setting means it will take longer to read a small

number of tags, but it will take a larger number of

transponders to saturate the communication

channel.

Table 7 below compares reading times at 4 kbit/s

vs. the number of transponders in a group.

each case the repeat delay was optimised for a

group of 30 transponders.

Time (s)

No of transponders

100

300

Free-running

3.1

5.8

10.8 49.3

Slow-down

0.86

1.8

5.8

Switch-off

0.79

1.5

3.4

Fast Free-running

0.30

0.78

2.9

690

Fast Slow-down

0.27

0.55

1.4

6.2

Fast Switch-off

0.26

0.49

1.0

3.3

Table 7

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