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Protocol Configuration
DH-485 Communication
Protocol
Publication 1763-RM001C-EN-P - October 2009
The information in this section describes the DH-485 network functions,
network architecture, and performance characteristics. It also helps you
plan and operate the controller on a DH-485 network.
DH-485 Network Description
The DH-485 protocol defines the communication between multiple
devices that coexist on a single pair of wires. DH-485 protocol uses
RS-485 Half-Duplex as its physical interface. (RS-485 is a definition of
electrical characteristics; it is not a protocol.) RS-485 uses devices that are
capable of co-existing on a common data circuit, thus allowing data to be
easily shared between devices.
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. To determine which initiator has the right to transmit, a token
passing algorithm is used.
The following section describes the protocol used to control message
transfers on the DH-485 network.
DH-485 Token Rotation
A node holding the token can send a message onto the network. Each
node is allowed a fixed number of transmissions (based on the Token
Hold Factor) each time it receives the token. After a node sends a
message, it passes the token to the next device.
The allowable range of node addresses 0 to 31. There must be at least one
initiator on the network (such as a MicroLogix controller, or an SLC 5/02
or higher processor).
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