EM4022V20WS11 datasheet(12/15 Pages) EMMICRO

12 / 15 page

 2002, EM Microelectronic-Marin SA

www.emmicroelectronic.com

EM4022

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, 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 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.

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

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.

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.

Figure 14 and 15 below show's reading times for some

possible versions

Optimum repeat delay settings

The following table lists the optimum repeat delay

settings for each of the protocols vx number of

transponders in a group.

Protocol

Number of transponders

100

Free-running

16k

64k

Slow-down

16k

Switch-off

16k

Fast Free-running

256

Fast Slow-down

256

Fast Switch-off

256

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