Rockwell-automation 1761 MicroLogix 1000 Programmable Controllers Manual de usuario descargar pdf (Pagina 339)

100

101

Understanding the Communication Protocols

D–11

DH-485 Communication Protocol

The information in this section describes the DH-485 network functions, network

architecture, and performance characteristics. It will also help you plan and operate

the MicroLogix 1000 on a DH-485 network.

Note Only

Series C or later Micr

oLogix 1000 discr

ete contr

ollers and all Micr

oLogix

1000 analog contr

ollers support the DH-485 network.

DH-485 Network Description

The DH-485 protocol defines the communication between multiple devices that

co-exist on a single pair of wires. This protocol uses RS-485 half-duplex as its

physical interface. (RS-485 is a definition of electrical characteristics; it is not

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.

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