This manual focuses on the main issues of troubleshooting the industrial data communications network of today, enabling you to walk onto your plant or facility to troubleshoot and fix problems as quickly as possible. The focus is on the Modbus suite of protocols and their associated standards. MORE INFORMATION: http://www.idc-online.com/content/practical-troubleshooting-and-problem-solving-modbus-protocols-196?id=84

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Practical Troubleshooting & Problem
Solving of Modbus Protocols

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Objectives
• Describe the modern instrumentation and control system
• List the main industrial communications systems
• Describe the essential components of industrial communications
systems

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Introduction
• Data communications - transfer of information from one point to another
• Modern instrumentation - Transfer of digital data
• Communications system
A transmitter to send information, a
receiver to accept it, and a link (the
medium) between the two
• Protocols - Rules governing the communication

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Modern instrumentation and control
systems
• Purpose of an instrumentation and control system :
Control of the processes and alarms
Control of sequencing, interlocking and alarms
An operator interface for display and control
Management information
• Devices having significant impact
on control of plants :
Distributed Control Systems (DCSs)
Programmable Logic Controllers (PLCs)
SCADA (Supervisory Control And Data Acquisition)
Smart Instruments

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Smart Instrumentation systems
Graphical representation of data communication

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OSI Model
• Developed by the International
Organization for Standardization
• Universal framework for building
communications systems
• 7-layer model
• Missing the 8th (‘user’) layer

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Open Systems Interconnection

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Protocols
• Rules for transmission of data
• Asynchronous (byte oriented) or synchronous (packet
oriented)
• Information can include:
– Sync byte(s)
– Function code
– Destination address
– Source address
– Data
– Checksum (BCC, CRC, etc)

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Basic structure of an information
frame
 SYNCHRONIZING BYTE(s) used to synchronize the receiver
 DESTINATION ADDRESS where the packet is going
 SOURCE ADDRESS where the packet is coming from
 DATA the information contained in the packet
 ERROR DETECTION (CHRECKSUM) BYTE(s) the result of the error
detection calculation

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Standards within a communications
system
• Codes (Hex, ASCII, etc)
• Physical layer implementations (RS-232, RS-485 etc)
• Protocols (Modbus, DF-1, etc)
• Networks (Profibus, Hart, AS-i, Data Highway Plus, Ethernet, etc)
A typical serial data communications link

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Typical two-wire multi-drop RS-485
network

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Standards
• Modbus :
Format of Modbus Serial message frame
• TCP/IP :
The Process/Application layer
The Services or Host-to-host layer
Internet layer
• Ethernet : Non-deterministic method, simple and low cost

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Radio modem configuration

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