Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 339)

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 MicroLogix 1000 discrete controllers and all MicroLogix
1000 analog controllers 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 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.