User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 12 of 33 July 27, 1999
4.1.2. Bit By Bit Interrogation
Upon power up the microprocessor waits for the next “wakeup word”. If there would be no ‘unnamed” tags
in the field the next word would be “wakeup + respond word”. However in this case, there is at least one
“unnamed” tag in the field and this will change Controller’s response.
The new Tag (or Tags if there is more then one new Tag) will respond with the pulse (“blip”) immediately
after it receives first 8 bits (9 pulses) from the Controller. When the Controller receives this “blip”, it
knows there is an unknown Tag in the field and instead of continuing with the “RESPOND” command
(0011), it sends “BIT BY BIT” command (0110). Only “unnamed” Tags will find this command of interest
and prepare for the BBB interrogation that follows.
The purpose of this interrogation is to overcome collision problems among different Tags if they try to
communicate to the Controller at the same time. In total 24 bits (3 bytes) need to be transferred from the
Tag to the Controller. This is Tag’s ID number.
One and only one Tag will finish interrogation and it will be the Tag with the highest ID number among all
“unnamed” Tags. All other Tags will, one by one, drop out of this interrogation after they try to send the ’0’
bit when there is a Tag which is sending the ‘1’ bit at the same time. This is the main principle of the BBB
interrogation and detailed description of this protocol can be found in the document “Bit By Bit
Interrogation: Protocol Description, EXI, 1999”.
Figure 2 Wakeup Word
‘1’
‘0’
2 ms
3 ms
500
µ
s
‘0’
‘0’
‘1’
‘1’
‘1’
‘1’
‘1’
‘0’
WAKEUP
‘0’
‘0’
‘1’
‘1’
‘1’
‘1’
‘0’
‘1’
‘0’
‘1’
RESPOND
WAKEUP
WORD
WAKEUP + RESPOND
WORD
100 ms 100 ms
WAKEUP
WAKEUP
WAKEUP
WAKEUP
+
RESPOND
WAKEUP
+
RESPOND
WAKEUP
+
RESPOND