Rockwell Automation 1784-KTXD User Manual download pdf (Page 184)

Publication 1747-UM011F-EN-P - May 2007

184 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.

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Rockwell Automation 1784-KTXD User Manual Page 184