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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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