Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 107)

Appendix A
Setting Up the DH-485 Network
A–2
A node holding the token can send any valid packet onto the network. Each
node is allowed only one transmission (plus two retries) each time it receives
the token. After a node sends one message packet, it attempts to give the
token to its successor by sending a “token pass” packet to its successor.
If no network activity occurs, the initiator sends the token pass packet again.
After two retries (a total of three tries) the initiator will attempt to find a new
successor.
Important: The maximum address that the initiator will search for before
wrapping to zero is the value in the configurable parameter
“maximum node address.” The default value for this parameter
is 31 for all initiators and responders.
The allowable range of the node address of an initiator is 0 to 31. The
allowable address range for all responders is 1 to 31. There must be at least
one initiator on the network.
Network initialization begins when a period of inactivity exceeding the time
of a link dead timeout is detected by an initiator on the network. When the
time for a link dead timeout is exceeded, usually the initiator with the lowest
address claims the token. When an initiator has the token it will begin to
build the network. The network requires at least one initiator to initialize it.
Building a network begins when the initiator that claimed the token tries to
pass the token to the successor node. If the attempt to pass the token fails, or
if the initiator has no established successor (for example, when it powers up),
it begins a linear search for a successor starting with the node above it in the
addressing.
When the initiator finds another active initiator, it passes the token to that
node, which repeats the process until the token is passed all the way around
the network to the first node. At this point, the network is in a state of
normal operation.
DH-485 Token Rotation
DH-485 Network Initialization