Dh-485 Communications; Dh-485 Network Protocol - Allen-Bradley SLC 500 Series Reference Manual
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SLC Communication Channels
DH-485 Communications
Publication 1747-RM001G-EN-P - November 2008
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 (4,000 ft).
DH-485 Network Protocol
The following section describes the protocol used to control message transfers
on the DH-485 network. The protocol 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 Token Rotation
A node holding the token can send valid packets onto the data link. The token
hold parameter determines the number of transmissions allowed (plus retries)
each time the node 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. If no network activity occurs, the
initiator attempts to find a new successor.
The node address range for an initiator is 0 to 31. The node address range for
all responders is 1 to 31. There must be at least one initiator on the network.
The maximum address that the initiator searches for before
TIP
wrapping to zero is the value in the configurable parameter
maximum node address. The default value of this parameter
is 31 for all initiators and responders.
SLC 500 processors do not allow node address zero to be
applied. If you attempt to apply a zero, node address one
becomes the processor node address. Node address zero is
reserved for a programming device, such as the Hand-Held
Terminal (HHT) or personal computer running
programming software.
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