EIT IDC Modbus Data Comms Systems

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Modbus Datacommunications

systems

bySteve Mackay

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EIT Micro-Course Series

• Every two weeks we present a 35to 45 minute interactive course

• Practical, useful with Q & Athroughout

• PID loop Tuning / Arc FlashProtection, Functional Safety,Troubleshooting conveyors

presented so far• Upcoming:

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• You get the recording and slides

Modbus Data Communication Systems

IDC Technologies and EIT



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Overall PresentationA review of the Modbus Protocol andrelated some troubleshooting issues.

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Objectives

• Give an introduction to Modbus

• Review simple troubleshooting steps





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Topics

• Introduction to Modbusprotocol

• TroubleshootingModbus

• Conclusion

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1.0 Introduction

to Modbus





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Objective• The Modbus Messaging protocol

• The transportation of Modbus requests and responses on serialnetworks

• Modbus memory allocation

• Modbus Function Codes and their application

• The difference between Modbus RTU and ASCII

• Troubleshooting Modbus systems in terms of:

No response to requests

Exception responses

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Modbus messaging

• Application layer (OSI layer 7) protocolthat provides client/server communicationbetween devices connected to different

types of buses or networks

• ‘Request/Response’ type protocol and nota ‘master/slave’ type protocol







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Modbus Serial communicationstack

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Protocol stack





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Modbus Serial ADU

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Modbus....

• The Function field

• The Data field

Allocation of Modicon memory address blocks

• The Address field

• The Error Check field





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RTU mode• 8 data bits, even, odd or no parity, 1 or 2 stop bits• Gap of 3 ½ character lengths used for header

(3 ½ mS)• Same for trailer• Gap in transmission >1,5ms causes frame to be discarded• CRC error checking• 999810 transmitted as <0x27><0x0E> (2 bytes)

RTU message frame

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ASCII Mode

• 7 data bits, even, odd or no parity, 1 or 2 stop bits• Header = : (colon)• Trailer = <CR> <LF>• Gaps of 1 second in frame acceptable• LRC error checking (not CRC)• 999810 (0x270E) transmitted as the ASCII string 270E i.e.

<0x32><0x37><0x30><0x45> (4 bytes)

ASCII message frame





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Function Code

• Read input discrete :

• FC01- reading coils-RTU mode :

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Function Codes....

• Read multiple registers :

• FC04-reading input register





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Function Codes....

• Write single register :

• Write coil :

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Force multiple coils

• Write multiple registers :





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2.0 Modbus - Troubleshooting

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Typical problems

• Hardware or software problems :

Mis-wired communication cabling and faultycommunication interfaces

Controller application tries to accessnon-existent target devices nodes oruse invalid Function Codes, addressnon-existent memory locations in thetarget devices, or specify illegal dataformat types

• Hardware problems

• Software (protocol)related problems





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Tools usedRS-232 breakout boxes, RS-232 to RS-485converters, continuity testers, voltmeters,screwdrivers, pliers, crimping tools andcabling tools.

• Hardware tools :

• Software tools Protocol analyzer

• Software troubleshooting

• Hardware troubleshooting

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Overall Concept





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Tools of the trade• Cable tester

• TCP/IP (and other third party) Utilities

• Protocol Analyzer (e.g. Wireshark)

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Hardware and Physical Cabling

• Check cable on both sides of connection

• Switch lights

• Incorrect wire type and components

• Straight versus cross over cable

• Excessive untwists

• Damaged RJ-45 connector

• Electrical noise and shielding





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TCP/IP Utilities• Ping

• Arp

• Netstat

• Nbtstat

• Ipconfig

• Tracert

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Ping -t• Easiest ways to test connectivity across the network and

confirm that an IP address is reachable





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Arp -a• Displays hardware and IP address

mapping

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Netstat -e

• Displays activitiessince last reboot

• Useful data on any

problems





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Nbtstat• Protocol Stats & TCP/IP connections

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Ipconfig /all





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Tracert

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Packet Analysis





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OSI Model

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Packet Structure





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Section 3Conclusion

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Thank You For Your Interest

If you are interested in further training, please visit:

IDC Technologies Short Courses:

Two-day practical courses available to the public:

http://idc-online.com/workshops

IDC Technologies Conferences:

http://idc-online.com/conferences-main

The Engineering Institute of TechnologiesOnline Certificate and

Advanced Diploma programs:www.eit.edu.au


EIT IDC Modbus Data Comms Systems

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