Publication 1764-UM001B-EN-P - April 2002
Understanding Communication Protocols E-5
Radio Modems
Radio modems may be implemented in a point-to-point topology
supporting either Half-Duplex or Full-Duplex communications, or in a
multi-drop topology supporting Half-Duplex communications
between three or more modems.
Line Drivers
Line drivers, also called short-haul “modems”, do not actually
modulate the serial data, but rather condition the electrical signals to
operate reliably over long transmission distances (up to several miles).
Line drivers are available in Full- and Half-Duplex models.
Allen-Bradley’s AIC+ Advanced Interface Converter is a Half-Duplex
line driver that converts an RS-232 electrical signal into an RS-485
electrical signal, increasing the signal transmission distance from 50 to
4000 feet (8000 feet when bridged).
DH-485 Communication
Protocol
The information in this section describes DH-485 network functions,
network architecture, and performance characteristics. It will also help
you plan and operate the MicroLogix controllers 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 (peer-to-peer) capability
• token passing access control
• the ability to add or remove nodes without disrupting the
network
• maximum network segment of 1219 m (4000 ft)
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