modbus data communication systems - eit

23/08/2013

1

www.eit.edu.au

Modbus Data communications

systems

by

Steve Mackay

www.eit.edu.au

EIT Micro-Course Series

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

• Practical, useful with Q & A throughout

• PID loop Tuning / Arc Flash Protection, Functional Safety, Troubleshooting conveyors presented so far

• Upcoming: – Electrical Troubleshooting and

much much more…..

• Go to http://www.eit.edu.au/free-courses

• You get the recording and slides

Engineering Institute of Technology (EIT)

Modbus Data Communication Systems Webinar

23/08/2013

2

www.eit.edu.au

Overall Presentation

A review of the Modbus Protocol and

related some troubleshooting issues.

www.eit.edu.au

Objectives

• Give an introduction to Modbus

• Review simple troubleshooting steps



23/08/2013

3

www.eit.edu.au

Topics

• Introduction to Modbus protocol

• Troubleshooting Modbus

• Conclusion

www.eit.edu.au

1.0 Introduction to Modbus



23/08/2013

4

www.eit.edu.au

Objective

• The Modbus Messaging protocol

• The transportation of Modbus requests and responses on serial

networks

• 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

www.eit.edu.au

Modbus messaging

• Application layer (OSI layer 7) protocol

that provides client/server communication

between devices connected to different

types of buses or networks

• ‘Request/Response’ type protocol and not

a ‘master/slave’ type protocol



23/08/2013

5

www.eit.edu.au

Master-slave interaction

www.eit.edu.au

Modbus transaction



23/08/2013

6

www.eit.edu.au

Modbus Serial communication stack

www.eit.edu.au

Protocol stack



23/08/2013

7

www.eit.edu.au

Modbus Serial ADU

www.eit.edu.au

Modbus....

• The Function field

• The Data field

Allocation of Modicon memory address blocks

• The Address field

• The Error Check field



23/08/2013

8

www.eit.edu.au

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

www.eit.edu.au

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



23/08/2013

9

www.eit.edu.au

Function Code

• Read input discrete :

• FC01- reading coils-RTU mode :

www.eit.edu.au

Function Codes....

• Read multiple registers :

• FC04-reading input register



23/08/2013

10

www.eit.edu.au

Function Codes....

• Write single register :

• Write coil :

www.eit.edu.au

Force multiple coils

• Write multiple registers :



23/08/2013

11

www.eit.edu.au

2.0 Modbus - Troubleshooting

www.eit.edu.au

Typical problems

• Hardware or software problems :

Mis-wired communication cabling and faulty

communication interfaces

Controller application tries to access

non-existent target devices nodes or

use invalid Function Codes, address

non-existent memory locations in the

target devices, or specify illegal data

format types

• Hardware problems

• Software (protocol)

related problems



23/08/2013

12

www.eit.edu.au

Tools used

RS-232 breakout boxes, RS-232 to RS-485

converters, continuity testers, voltmeters,

screwdrivers, pliers, crimping tools and

cabling tools.

• Hardware tools :

• Software tools Protocol analyzer

• Software troubleshooting

• Hardware troubleshooting

www.eit.edu.au

Overall Concept



23/08/2013

13

www.eit.edu.au

Tools of the trade

• Cable tester

• TCP/IP (and other third party) Utilities

• Protocol Analyzer (e.g. Wireshark)

www.eit.edu.au

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



23/08/2013

14

www.eit.edu.au

TCP/IP Utilities• Ping

• Arp

• Netstat

• Nbtstat

• Ipconfig

• Tracert

www.eit.edu.au

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

confirm that an IP address is reachable



23/08/2013

15

www.eit.edu.au

Arp -a

• Displays hardware and IP address

mapping

www.eit.edu.au

Netstat -e

• Displays activities since last reboot

• Useful data on any problems



23/08/2013

16

www.eit.edu.au

Nbtstat

• Protocol Stats & TCP/IP connections

www.eit.edu.au

Ipconfig /all



23/08/2013

17

www.eit.edu.au

Tracert

www.eit.edu.au

Packet Analysis



23/08/2013

18

www.eit.edu.au

OSI Model

www.eit.edu.au

Packet Structure



23/08/2013

19

www.eit.edu.au

Section 3Conclusion

www.eit.edu.au

Thank You For Your InterestIf 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



modbus data communication systems - eit

Documents