Rockwell-automation 1763 MicroLogix 1100 Programmable Controllers User Manual de usuario descargar pdf (Pagina 207)

Publication 1763-UM001D-EN-P - March 2011

Connecting to Networks via RS-232/RS-485 Interface 205

DH-485 Communication

Protocol

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 (peer-to-peer) capability

• token passing access control

• the ability to add or remove nodes without disrupting the network

• maximum network segment of 1,219 m (4,000 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.

Control of message transfers on the DH-485 network is performed by rotating

the token along the nodes on the network. 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 is 1 to 31. There must be at least one

initiator on the network (such as a MicroLogix controller, or an SLC 5/02 or

later processor).

DH-485 Configuration Parameters

When MicroLogix communications are configured for DH-485, the following

parameters can be changed:

See Software Considerations on page 208 for tips on setting the parameters

listed above.

DF1 Full-Duplex Configuration Parameters

Parameter Options

Baud Rate 9600, 19.2K

Node Address 1 to 31 decimal

Token Hold Factor 1 to 4

1 2 ... 202 203 204 205 206 207 208 209 210 211 212 ... 255 256

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Rockwell-automation 1763 MicroLogix 1100 Programmable Controllers User Manual de usuario Pagina 207