User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 11 of 33 July 27, 1999
4. RF Communication Protocol
All communication between the Tags and the Controllers is taking place across RF communication media.
Two frequencies are used:433.92 MHz and 307 kHz.
• 433.92 MHz Communication
This is a frequency which is used to transfer information from the Tag to the Controller. The guaranteed
range is 30 feet.
• 307 kHz Communication
This is a “field of coverage” which is modulated in order to send information from the Controller to the
Tag. The range of this field depends on a particular application and is usually set by the installer but it can
not exceed 10 feet.
There are two types of information that need to be transferred from the Tag to the Controller, “Tag in the
Field” (TIF) alarm and “Tag Initiated Communication” (TIC) alarm. These two alarms are actually the
essence of the whole “Halo” system.
4.1. Tag in the Field (TIF Alarm)
The Controller needs to know when any Tag enters the “field of coverage”. This information is then used to
take certain action or not, depending on the system setup.
4.1.1. Field of coverage
The Controller generates the “field of coverage” around itself continuously. It is transmitting “wakeup
word” every 100 ms and waits for the response from any potential Tags that could enter this field or the
Tags already in the field previously “named”.
A form of PWM is used to represent ‘zeros’ and ‘ones’. As shown in the Figure 2, a logical ‘1’ is
represented as 2 ms gap between pulses, and a logical ‘0’ as 3 ms gap between pulses. The pulse itself is
500 µs wide.
Figure 2 also illustrates how the “field of coverage” is formed. The Controller alternates between plain
“wakeup word” (97 Hex) and “wakeup + respond word” (97 + 03 Hex). Only the plain “wakeup word”
followed by 20 ms of silence (no pulses) can power up Tag’s processor but not “wakeup + respond word”.
This is important so that Controller wouldn’t be confused with the response it receives from the Tag, not
knowing is it the Tag just powered up or the Tag already “named”. Any pulse that comes within 20 ms after
the plain “wakeup word” (either from the noise or “respond” portion of the “wakeup + respond word” will
advance the hardware counter and microprocessor will not be powered up.
The Tag must receive all 9 pulses of the “wakeup word” and if it misses any of these pulses, the hardware
counter will reset and it will wait for the next plain “wakeup word”.
After it is powered up, the microprocessor will set one of its lines and maintain its own power supply. If
this line is not set within 40 ms it will be powered down again.