Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 93)

Publication 1757-UM006A-EN-P - May 2002
Configurating the 1757-FIM 4-7
Since the CN2FF only occasionally polls the addresses that have
shown no previous activity (called the Slow Poll List), it takes
relatively longer to detect a node at a new address than to pass the
Token to a known node at an active address. The sequence is sort of
like:
Pass Token 17
Pass Token 18
Pass Token 19
Pass Token 20
Probe Node 32
<no answer>
Pass Token 17
Pass Token 18
Pass Token 19
Pass Token 20
Probe Node 33
<no answer>
etc. up to
Probe Node 255
<no answer>
Pass Token 17
Pass Token 18
Pass Token 19
Pass Token 20
Probe Node 21
<answer from our new Node 21>
Pass Token 17
Pass Token 18
Pass Token 19
Pass Token 20
Pass Token 21 (this one is in the Live List now)
Probe Node 22
etc.
You can see that the token is passed around the ring perhaps 200
times between Probes of inactive addresses to see if there is someone
new.
Software Example:
Forever:
For each address not in the Live List
For each node in the Live List
Pass Token
<node transmits if it has something to say>
<node returns token>
end For
Probe Address to see if a Node is there now.
if Node answers, add to Live List
end For
End Forever