Allen-Bradley ControlLogix 1756-DH485 User manual

ControlLogix
DH-485
Communication
Module
1756-DH485
User Manual

Important User Information
Solid state equipment has operational characteristics differing from those of
electromechanical equipment. Safety Guidelines for the Application, Installation and
Maintenance of Solid State Controls (publication SGI-1.1 available from your local
Rockwell Automation sales office or online at
http://literature.rockwellautomation.com) describes some important differences
between solid state equipment and hard-wired electromechanical devices. Because of
this difference, and also because of the wide variety of uses for solid state equipment,
all persons responsible for applying this equipment must satisfy themselves that each
intended application of this equipment is acceptable.
In no event will Rockwell Automation, Inc. be responsible or liable for indirect or
consequential damages resulting from the use or application of this equipment.
The examples and diagrams in this manual are included solely for illustrative purposes.
Because of the many variables and requirements associated with any particular
installation, Rockwell Automation, Inc. cannot assume responsibility or liability for
actual use based on the examples and diagrams.
No patent liability is assumed by Rockwell Automation, Inc. with respect to use of
information, circuits, equipment, or software described in this manual.
Reproduction of the contents of this manual, in whole or in part, without written
permission of Rockwell Automation, Inc., is prohibited.
Throughout this manual, when necessary, we use notes to make you aware of safety
considerations.
The first line of trademarks is an example; the brand or product name changes according to publication but the rest of the line should remain the
same. The second line should be used exactly as listed below in any case where it applies.
Allen-Bradley, ControlLogix, and RSLinx are trademarks of Rockwell Automation, Inc.
Trademarks not belonging to Rockwell Automation are property of their respective companies.
WARNING Identifiesinformationaboutpracticesorcircumstancesthatcancause
an explosion in a hazardous environment, which may lead to personal
injury or death, property damage, or economic loss.
IMPORTANT Identifies information that is critical for successful application and
understanding of the product.
ATTENTION Identifies information about practices or circumstances that can lead
to personal injury or death, property damage, or economic loss.
Attentions help you identify a hazard, avoid a hazard, and recognize
the consequence
SHOCK HAZARD
Labelsmaybelocatedonorinsidetheequipment,forexample,adrive
or motor, to alert people that dangerous voltage may be present.
BURN HAZARD Labelsmaybelocatedonorinsidetheequipment,forexample,adrive
or motor, to alert people that surfaces may be dangerous
temperatures.

iPublication 1756-UM532A-EN-P - May 2006
Table of Contents
Important User Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
Preface Purpose of This Manual. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface-i
Who Should Use This Manual. . . . . . . . . . . . . . . . . . . . . . . . . . Preface-i
Other Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface-i
Chapter 1
1756-DH485 Module Overview
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
What the Module Does . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
Module Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
Module Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
Prevent Electrostatic Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
Removal and Insertion Under Power . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
Typical Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5
Remote Messaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7
Remote Upload and Download . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7
Routing Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8
DH-485 and CIP Messaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8
Alphanumeric Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9
Chapter 2
Overview of the DH-485 Network
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
DH-485 Network Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
DH-485 Network Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
DH-485 Token Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
DH-485 Network Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
Devices that Use the DH-485 Network . . . . . . . . . . . . . . . . . . . . . . . 2-3
1747-AIC Isolated Link Coupler for DH-485 . . . . . . . . . . . . . . . . . . 2-4
1761-NET-AIC Advanced Interface Converter Product Overview . 2-6
Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
Device Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
Node Address Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7
Misconception about the 1761-NET-AIC Converter . . . . . . . . . . . . 2-7
1747-UIC USB to DH-485 Interface Converter . . . . . . . . . . . . . . . . 2-7
Example System Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8
Configure the SLC 5/03, SLC 5/04, and SLC 5/05 Controller
Channel 0 for DH-485. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9
Important Planning Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 2-10
Hardware Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10
Software Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12
Additional Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13

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Table of Contents ii
Chapter 3
Use Data Highway 485 Network
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
What Is Data Highway 485? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
Link Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
Two Methods of Communication Over a DH-485 Network . . . . . . 3-2
Use DH-485 Messaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3
Local DH-485 Messaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3
Limitations of Local DH-485 Messaging . . . . . . . . . . . . . . . . . . . 3-5
Remote DH-485 Messaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
Configuration Information in DH-485 Messaging. . . . . . . . . . . . . . 3-11
Application Timeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12
Example DH-485 Network Routing Configuration. . . . . . . . . . . . . 3-13
Use the Common Industrial Protocol (CIP) Messaging. . . . . . . . . . 3-14
Chapter 4
Use RSLinx Software to Create a
Routing Table
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
Choose the Correct Software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
Use RSLinx Software to Create a Routing Table . . . . . . . . . . . . . . . . 4-2
What Is a Routing Table? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
ControlLogix Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
Create the Routing Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3
Set the Controller Slot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
Chapter 5
Communicate from an SLC 5/03
Controller to a Logix Controller
over a DH-485 Network
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
Use the Controller Slot to Route the Local Message . . . . . . . . . . . . . 5-3
Create a Local Message in RSLogix 500 . . . . . . . . . . . . . . . . . . . . . . . 5-4
Configure the Message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
Create a Logic Map for the Local Message in
RSLogix 5000 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

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Table of Contents iii
Chapter 6
Communicate Between SLC
Controllers over DH+ and DH485
Networks
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
SLC 5/04 Controller Sends a Remote Message . . . . . . . . . . . . . . . . . 6-2
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
Create the Routing Table in RSLinx Software . . . . . . . . . . . . . . . 6-3
Configure the SLC 5/03 Controller Channel 1 Link ID . . . . . . . 6-4
Configure the SLC 5/04 Channel 1 Link ID . . . . . . . . . . . . . . . . 6-5
Create a Message in RSLogix 500 . . . . . . . . . . . . . . . . . . . . . . . . . 6-6
Configure the Message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
SLC 5/03 Controller Sends a Remote Message . . . . . . . . . . . . . . . . . 6-8
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8
Create the Routing Table in RSLinx Software . . . . . . . . . . . . . . . 6-9
Create a Remote Message in RSLogix 500 . . . . . . . . . . . . . . . . . 6-10
Configure the Message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11
Chapter 7
Communicate Between a PLC-5
Controller and a Remote SLC 5/03
Controller over Multiple DH-485
Networks
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
Create the Routing Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
Configure the PLC-5 DH+ Channel 1A Link ID . . . . . . . . . . . . 7-4
Create a Message in RSLogix 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
Configure the Message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5
Chapter 8
Communicate from an SLC 5/05
Controller to an SLC 5/03
Controller over an EtherNet/IP
Network
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
Create a Message in RSLogix 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3
Configure the Message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4
Review the Multihop Feature in RSLogix 500 . . . . . . . . . . . . . . . . . . 8-5
Chapter 9
Communicate to a SLC 5/03
Controller on a DH-485 Network
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2
Create a Message in RSLogix 5000 . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3
Configure the Message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4

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Table of Contents iv
Chapter 10
Communicate from a Logix
Controller to an SLC 5/03
Controller over EtherNet/IP and
DH-485 Networks
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1
Hardware Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2
Create a Remote 1756-ENBT Message in RSLogix 5000 . . . . . . . . 10-3
Configure the Message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4
Appendix A
Specifications Appendix B
Troubleshoot the 1756-DH485
Module
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1
Check Power Supply and Module Status . . . . . . . . . . . . . . . . . . . . . . B-1
Interpret the LED Status Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . B-2
Interpret the Alphanumeric Indicators . . . . . . . . . . . . . . . . . . . . . . . . B-3
Interpret the OK
Status Indicator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-4
Routing Errors in DH-485 Messaging . . . . . . . . . . . . . . . . . . . . . . . . B-4
Routing Errors in Local DH-485 Messaging . . . . . . . . . . . . . . . . B-4
Routing Errors in Remote DH-485 Messaging . . . . . . . . . . . . . . B-5
Index
Rockwell Automation Support . . . . . . . . . . . . . . . . . . . . . . . . Backcover
Installation Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . Backcover
New Product Satisfaction Return . . . . . . . . . . . . . . . . . . . Backcover

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Preface
Purpose of This Manual
This manual describes how to understand, configure, and troubleshoot your
ControlLogix Data Highway 485 communication interface module.
This manual also provides step-by-step procedures on how to use the
1756-DH485 module to send DH-485 messages between ControlLogix, PLC,
and SLC controllers in DH-485 applications.
Who Should Use This
Manual
Use this manual if you program applications that use 1756-DH485 modules.
We assume you have a good understanding of the Data Highway 485 protocol.
This user manual contains a brief description of the Data Highway 485
network in Chapter 2, Overview of the DH-485 Network.
Other Resources
Refer to the manuals listed in the table Devices that Use the DH-485 Network
on page 2-3.

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Preface ii
Notes:

1Publication 1756-UM532A-EN-P - May 2006
Chapter 1
1756-DH485 Module Overview
Introduction
This chapter describes the 1756-DH485 module, and what you must know and
do before you begin to use it.
What the Module Does
The 1756-DH485 module supports the following types of communication:
•Data Highway 485 (DH-485) Messaging
•Connectivity to two separate DH-485 networks in one module
•Common Industrial Protocol (CIP) Messaging
Module Requirements
The following are required for using the 1756-DH485 communication
module:
•RSLinx Software, version 2.41 or later
•Node address within the range of 0-31
•AIC+ link coupler (1761-NET-AIC) communications adapter
•Compatible with SLC 5/03 controllers or later
For See
What the Module Does 1-1
Module Requirements 1-1
Module Features 1-2
Prevent Electrostatic Discharge 1-3
Removal and Insertion Under Power 1-4
Typical Applications 1-5
Remote Messaging 1-7
Routing Limitations 1-8

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1-2 1756-DH485 Module Overview
Module Features
The 1756-DH-485 module offers the following features:
•Sends messages between devices on DH-485 networks and devices on
other networks such as ControlNet, EtherNet/IP, or other DH-485
networks
•Bridges support to other NetLinx networks via the CLX gateway
•Configure channels and ports using RSLinx Classic software, version
2.43 or later
•Accesses other networks by using a routing table editor that lets
DH-485 devices use the 1756-DH485 module and ControlLogix chassis
•Supports all benefits and attributes of a ControlLogix communication
module
•Supports firmware upgrades
•Removal and insertion under power (RIUP), can be removed and
inserted under power without disrupting power to other modules in the
chassis
•4-character display
•Default Controller Slot feature for local messaging
•Serial Port Configuration
•No limit on number of modules per chassis, up to the number of
available slots and the capabilities of the power supply
This figure shows the external features of the 1756-DH485 module.
TIP Download firmware upgrades via the backplane. Do
not use DH485 network for downloads.
WARNING
!
When you insert or remove the module while backplane
power is on, an electrical arc can occur. This could cause an
explosion in hazardous location installations.
Be sure that power is removed or the area is nonhazardous
before proceeding. Repeated electrical arcing causes
excessive wear to contacts on both the module and its
mating connector. Worn contacts may create electrical
resistance that can affect module operation.

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1756-DH485 Module Overview 1-3
External Features of the 1756-DH485 Module
Prevent Electrostatic
Discharge
The DH-485 module is sensitive to electrostatic discharge.
Channel B
Connector
Channel A
Connector
Channel
and
Module
Status
Indicators
43842
Backplane
Connector
Alphanumeric
Status Indicator Door
Label
ATTENTION
!
This equipment is sensitive to electrostatic discharge, which
can cause internal damage and affect normal operation.
Follow these guidelines when you handle this equipment:
•Touch a grounded object to discharge potential static.
•Wear an approved grounding wriststrap.
•Do not touch connectors or pins on component
boards.
•Do not touch circuit components inside the equipment.
•Use a static-safe workstation, if available.
•Store the equipment in appropriate static-safe
packaging when not in use,.

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1-4 1756-DH485 Module Overview
Removal and Insertion
Under Power
You can install or remove the module while chassis power is applied if you
observe the following precautions.
WARNING
!
When you insert or remove the module while backplane
power is on, an electrical arc can occur. This could cause an
explosion in hazardous location installations.
Be sure that power is removed or the area is nonhazardous
before proceeding. Repeated electrical arcing causes
excessive wear to contacts on both the module and its
mating connector. Worn contacts may create electrical
resistance that can affect module operation.

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1756-DH485 Module Overview 1-5
Typical Applications
The first example is a typical gateway application connecting multiple networks
together.
In this example you can:
•upload and download SLC 500 and Panelview display programs.
•seamlessly communicate between SLC controllers.
•connect multiple DH-485 networks together for inter-network
communications, for example, messages between SLC 5/03 controllers.
•provide connection to other NetLinx networks, such as EtherNet/IP, to
allow a single-point access, for example: upload and download to an
SLC controller, MicroLogix controller, and Panelview display.
Connection of Multiple Networks
44137
1756-DH485
AIC+ Link Couplers
PanelView
SLC 5/03 Controller
SLC 5/03 Controller
Computer with RSlogix 500
and RSLogix 5000 Software
1756-ENBT
AIC Link Coupler
AIC Link Coupler AIC Link Coupler

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1-6 1756-DH485 Module Overview
The second example shows how you can connect or migrate a legacy system to
a new installation with RSLogix software.
•Connect multiple DH-485 networks together for inter-network
communications.
•Message between SLC 5/03 controllers.
•Provide connection to other NetLinx networks.
•Access through a single point and download to an SLC controller,
MicroLogix controller, and Panelview display.
•Allow communications between legacy and ControlLogix systems for
high integration of control systems (except for fixed SLC controllers).
•Phase SLC systems into ControlLogix architecture by using a migration
tool.
Connect or Migrate a Legacy System to a New Installation
EXTERNAL
X
EXTERNAL
X
TERM
T
A
A
A
44136
1756-DH485
AIC+ Link Coupler
AIC Link Coupler
PanelView Display
SLC 5/03 Controller
1756-ENBT Module
AIC+ Link Coupler
AIC Link Coupler
SLC 5/03 Controller
MicroLogix
DH485 Network
ControlLogix Controller
ControlLogix Controller
Computer with RSlogix 500
and RSLogix 5000 Software
AIC Link Coupler
AIC+

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1756-DH485 Module Overview 1-7
Remote Messaging
The configuration of remote messaging is similar to data highway remote I/O
(DHRIO). It is achieved through routing table functions and module
configuration. The multi-hop functionality is available in RSLogix 5 and 500
(version 6.3 or later)
Remote Upload and Download
You can bridge EtherNet/IP and ControlNet networks to a DH-485 network
using the 1756-DH485 module
You can upload and download:
•all MicroLogix and SLC 500 controllers on DH-485 from NetLinx
network by using RSLinx Classic software.
•PanelView Standard terminals on DH-485 from NetLinx network by
using RSLinx Classic.
Logix controllers message via a NetLinx network through a ControlLogix
gateway to the DH-485 network to all MicroLogix and SLC 5/03 (and above)
controllers on DH-485 network. RSLinx Classic software on the DH-485
network can browse through the ControlLogix gateway to any device on a
NetLinx network.
IMPORTANT Logix controllers on a NetLinx network will not be able to
initiate messages through the ControlLogix gateway to the
DH-485 network to fixed SLC 500, SLC 5/01, or SLC
5/02 controllers on a DH-485 network.

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1-8 1756-DH485 Module Overview
Routing Limitations
The 1756-DH485 module can route a message through up to four
communications networks and three chassis. This limit applies only to the
routing of a message and not to the total number of networks or chassis in a
system.
DH-485 and CIP Messaging
The 1756-DH485 module lets devices, such as ControlLogix, PLC, and SLC
controllers exchange information. With the 1756-DH485 module, you may
exchange information between the ControlLogix, PLC, or SLC controllers on
different networks. Communication between SLC controllers on different
networks is accomplished using remote messaging.
Communication Between SLC Controllers on Different Networks
44056
Programming Terminal
ControlLogix
Chassis
ControlLogix
Chassis
DH-485
DH-485
SLC 5/03 PanelView
Data Collection and
Recipe Management RSView
MicroLogix SLC 5/03
PanelView
EtherNet/IP Network

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1756-DH485 Module Overview 1-9
Alphanumeric Indicators
When you apply power to the module, the alphanumeric display begins to
cycle through the following sequence.
1. All LEDs flash on then off -CHA, CHB OK
2. OK displays red then changes to green.
3. INIT displays.
4. Firmware Revision flashes on scrolling display: DH-485
5. Channel A and the network used for channel A.
6. Channel A node address.
7. Channel A status.
8. Channel B and the network used for channel.
9. Channel B node address.
10. Channel B status.
This sequence runs continuously during normal module operation.
Refer to Appendix B, Troubleshoot the 1756-DH485 Module and publication
1756-IN587, the ControlLogix DH-485 Communications Module Installation
Instructions, for more information about LEDs and status indicators.
EXAMPLE For example, if your module uses the following:
•Channel A for DH-485 with node address 14
•Channel B is not connected
and the channels are operating properly, you see the
following sequence:
A DH, A#14, A OK, B DH, B#00, ONLY NODE

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Notes:

1Publication 1756-UM532A-EN-P - May 2006
Chapter 2
Overview of the DH-485 Network
Introduction
The information in this chapter will help you plan, install, and operate devices
on a DH-485 network. This chapter also contains information that describes
the DH-485 network functions, network architecture, and performance
characteristics.
DH-485 Network
Description
The DH-485 network passes information between devices on the plant floor.
The network monitors process parameters, device parameters, device status,
process status, and application programs to support data acquisition, data
monitoring, program upload/download, and supervisory control.
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)(1).
For See
DH-485 Network Description 2-1
DH-485 Network Protocol 2-2
DH-485 Token Rotation 2-2
DH-485 Network Initialization 2-2
Devices that Use the DH-485 Network 2-3
1747-AIC Isolated Link Coupler for DH-485 2-4
1761-NET-AIC Advanced Interface Converter Product Overview 2-6
Operating Modes 2-6
Device Compatibility 2-6
1747-UIC USB to DH-485 Interface Converter 2-7
Example System Configuration 2-8
Important Planning Considerations 2-10
Additional Resources 2-13
(1) The network can be extended to 2,438meters (8,000 feet) by connecting two AIC+ Advanced Interface
Converters (1761-NET-AIC). Refer to the AIC+ Advanced Interface Converter User Manual, publication number
1761-6.4.

Publication 1756-UM532A-EN-P - May 2006
2-2 Overview of the DH-485 Network
The protocol used to control message transfers on the DH-485 network
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.
DH-485 Network Protocol
A node holding the token is able to transmit a configurable number of
message packets (plus two retries) onto the network before having to pass the
token on to the next node. This configuration parameter is called the Token
Hold Factor and has a minimum value of one and a maximum value of four.
Once a node holding the token has sent any messages that it has to send, up to
its Token Hold Factor maximum allowed, it attempts to pass the token to its
successor by sending a token-pass packet.
DH-485 Token Rotation
If no network activity occurs, the initiator sends the token-pass packet again.
After two retries (a total of three tries) the initiator will attempt to find a new
successor.
The allowable range of the node address of an initiator is 0 to 31. The
allowable address range for all responders is 1 to 31. There must be at least one
initiator on the network.
DH-485 Network
Initialization
Network initialization begins when a period of inactivity exceeding the time of
a link dead timeout is detected by an initiator on the network. When the time
for a link dead timeout is exceeded, usually the initiator with the lowest address
claims the token. When an initiator has the token it will begin to build the
network. The network requires at least one initiator to initialize it.
Building a network begins when the initiator that claimed the token tries to
pass the token to the successor node. If the attempt to pass the token fails, or
if the initiator has no established successor (for example, when it powers up), it
begins a linear search for a successor starting with the node above it in the
addressing.
When the initiator finds another active initiator, it passes the token to that
node, which repeats the process until the token is passed all the way around
the network to the first node. At this point, the network is in a state of normal
operation.
IMPORTAN
T
The maximum address that the initiator will search for
before wrapping to zero is the value in the configurable
parameter maximum node address. The default value for
this parameter is 31 for all initiators and responders.