ALLEN BRADLEY 1747-L551 PLC Processor

A–2 Setting Up the DH-485 Network
DH-485 Token Rotation
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.
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.
Devices that use the
DH-485 Network
Presently, the following SLC 500 devices support the DH-485
network:
• SLC 500 Fixed I/O Controller (responder)
• SLC 5/01 Modular I/O Controller (responder)
• SLC 5/02 Modular I/O Controller (initiator/responder)
• SLC 5/03 Modular I/O Controller (initiator/responder)
• SLC 5/04 Modular I/O Controller (initiator/responder)
• SLC 5/05 Modular I/O Controller (initiator/responder)
• Personal computer running your programming software (initiator)
• Hand-Held Terminal (initiator)
• DTAM (initiator/responder)
Publication 1747-6.2