Rockwell Automation 5720 DriveLogix System - Manual (Page 215)

Publication 20D-UM002C-EN-P - November 2003
Communicating with Devices on a DH-485 Link 10-5
If no network activity occurs, the initiator sends the token pass packet again.
After two retries (a total of three tries) the initiator attempts to find a new
successor.
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
The network requires at least one initiator to initialize it. Network initialization
begins when an initiator on the network detects a period of inactivity that
exceeds the time of a link dead timeout. When the 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.
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.
Number of nodes and node addresses
The number of nodes on the network directly affects the data transfer time
between nodes. Unnecessary nodes (such as a second programming terminal
that is not being used) slow the data transfer rate. The maximum number of
nodes on the network is 32.
If the node addresses for controllers are assigned in sequence, starting at node
1 (with node 0 left for a programming terminal), it is as efficient to leave the
maximum node address at 31 as it is to decrease it to the highest node address
on the network. Then, adding devices to the network at a later time will not
require modifying the maximum node address in every device on the network.
IMPORTANT
The maximum address that the initiator searches for before
starting again with zero is the value in the configurable
parameter “maximum node address.” The default value for
this parameter is 31 for all initiators and responders.