User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 13 of 33 July 27, 1999
After it transmits all 24 bits of the ID word (3 bytes are treated as one ID word) in the PWM format, the
Tag sends additional byte in a NRZ (non return to zero) format. This is a checksum of the previous ID word
which will be used by the Controller to ensure that the received ID number is correct.
The main characteristic of the NRZ code is that bits are represented by the duration of the logic state and
not by the logic state itself. A logic ‘1’ is a logic state with duration of 500 µs, and a logic ‘0’ lasts for 250
µs. More about this code can be found in the document “Bit By Bit Interrogation: Protocol Description,
EXI, 1999”.
The checksum is calculated by using “Cyclic Redundancy Code” and this code is describe in details in the
document “Halo” Cyclic Redundancy Check, Algorithm Implementation, EXI, 1999”.
Upon receiving the checksum byte, the Controller will issue either ACK (1010) response or REPEAT
(1111) response depending if calculated checksum has matched received checksum or not. If it receives
ACK response, the Tag knows that the ID number was successfully received by the Controller and it will
not respond anymore to the BBB command. This will enable the Tag with the next sequentially lower ID to
finish BBB interrogation next time.
If the Controller respond with REPEAT, the Tag knows that the checksum did not match and it will retry
next time when it receives “wakeup word”.
Figure 3 illustrates BBB interrogation.
4.1.3. Respond Command
The Tags which ID number has been retrieved by the Controller are called “named” tags. These tags will
respond to the “wakeup + respond word”, so that the Controller knows if there is anymore Tags in the field
and when they are all gone outside the field of coverage. If there is no “respond” pulses after “wakeup +
respond word” the Controller will terminate TIF alarm and return to the idle mode.
Figure 4 illustrates the Tags response to the “wakeup + respond word”. All tags present in the field and
“named” will respond with the same “blip” (350 µs wide).
Figure 3. Controller TX and RX signals
Wakeup Word
BBB
Command
Bit-By-Bit ACK
Data CRC
TX
RX
1 0 0 1 0 1 1 1 0 1 1 0 0 1 1 0 0 1 0 1 1 1 0 1 0
Blip
Figure 2 Controller TX and RX signals
‘1’
‘0’
WAKEUP
‘0’
‘0’
‘1’
‘1’
‘1’
‘1’
‘0’
‘1’
‘0’
‘1’
RESPOND
TX
RX
RESPOND