Rockwell Automation 1756-DH485 ControlLogix DH-485 Communication Module User Manual - Manual (Page 20)

Publication 1756-UM532A-EN-P - May 2006
2-2 Overview of the DH-485 Network
The protocol used to control message transfers on the DH-485 network
supports two classes of devices: initiators and responders. All initiators on the
network get 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 is able to transmit a configurable number of
message packets (plus two retries) onto the network before having to pass the
token on to the next node. This configuration parameter is called the Token
Hold Factor and has a minimum value of one and a maximum value of four.
Once a node holding the token has sent any messages that it has to send, up to
its Token Hold Factor maximum allowed, it attempts to pass the token to its
successor by sending a token-pass packet.
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 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.
DH-485 Network
Initialization 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.
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.