User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 15 of 33 July 27, 1999
4.2.1. Message format
The Tag (any of the above) is powered down most of the time and it powers up only when it enters the
“field of coverage” (TIF) or when it needs to generate the TIC alarm.
The TIC alarm range should be much longer than a TIF alarm and this is accomplished with some tradeoffs.
The most important factor is that TIC communication can not be 2-way communication due to the nature
of the Tags 2-way communication (field of coverage has limited range). This determines the format of the
TIC message sent from the Tag to the Controller.
To improve the chances that the message will go through (without the acknowledgment it can not be
guarantied nor the Tag knows if the message has been received correctly by the Controller), it is sent in 3
bursts and 5 times within each burst as shown in the Figure 5. This is 15 ID messages in total sent every
time when the TIC is generated. If any of these messages comes through, the transmission is successful.
After receiving each one of these ID messages the Controller compares received checksum with calculated
CRC and if they are the same, ignores all other messages with the same ID number for next 2 seconds.
The message is formatted in the NRZ code (same protocol as described earlier). Each individual ID
message consists of 3 ID bytes (coded as 6 BCD numbers) and one byte for the checksum (same CRC
calculation as described for “Bit-By-Bit” protocol).
After each burst (with 5 ID messages) the microprocessor goes into the sleep mode. The bursts are
generated every 850 ms (Figure 5a).
Figure 5b illustrates one of the bursts. The ID messages are separated with 2 ms of silence. This time is
used by the Controller to calculate the CRC.
The ID message format is shown in the Figure 5c. There are 4 bytes in the NRZ code where a logic ‘0’ is
represented with the 250 µs pulse and a logic ‘1’ with the 500 µs pulse. Each byte is terminated with the
“stop bit”. There is 1 ms gap between each byte.