Rockwell Automation Allen-Bradley DriveLogix 5730, User Manual

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9-4 Communicating with Devices on a DH485 Link
Specify these characteristics on the Serial Port tab (default values are shown
in bold):
Planning a DH-485 Network The DH-485 network offers:
•
interconnection of 32 devices
•
multi-master capability
•
token passing access control
•
the ability to add or remove nodes without disrupting the network
•
maximum network length of 1219 m (4000 ft.)
The DH-485 protocol supports two classes of devices: initiators and
responders. All initiators on the network get a chance to initiate message
transfers. The DH-485 protocol uses a token-pass algorithm to determine
which initiator has the right to transmit
DH-485 Token Rotation
A node holding the token can send any valid packet onto the network. Each
node gets 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 attempts to find a new
successor.
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.
Characteristic: Description (default is shown in bold):
Baud Rate Specifies the communication rate for the DH-485 port. All devices on the same DH-485 network
must be configured for the same baud rate. Select 9600 or
19200 Kbps.
Node Address Specifies the node address of the DriveLogix controller on the DH-485 network. Select a
number
1 -31 decimal, inclusive.
To optimize network performance, assign node addresses in sequential order. Initiators, such as
personal computers, should be assigned the lowest address numbers to minimize the time required
to initialize the network.
Token Hold Factor Number of transmissions (plus retries) that a node holding a token can send onto the data link each
time that it receives the token. Enter a value between 1-4. The default is 1.
Maximum Node Address Specifies the maximum node address of all the devices on the DH-485 network. Select a
number 1-
31 decimal, inclusive.
To optimize network performance, make sure:
the maximum node address is the highest node number being used on the network
that all the devices on the same DH-485 network have the same selection for the maximum node
address.