Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 182)

Publication 1747-UM011G-EN-P - June 2008
182 Setting Up the DH-485 Network
a chance to initiate message transfers. To determine which initiator
has the right to transmit, a token passing algorithm is used.
DH-485 Network Protocol 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.
DH-485 Token Rotation 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.
The allowable range of the node address of a initiator is 0...31. The
allowable address range for all responders is 1...31. There must be at
least one initiator on the network.
DH-485 Network
Initialization Network initialization begins when a period of inactivity exceeding
the time of a link dead timeout is detected by a 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 a 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 turns on), 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.
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.