Practical Industrial Data Networks:Design, Installation and ...

Modbus overview 1097.4 Troubleshooting7.4.1 Common problems and faultsNo matter what extremes of care that you may have taken, there is hardly ever aninstallation that boasts of trouble-free setup and configuration. Some common problemsrelated to Modbus installations are listed below. They can be categorized as eitherhardware or software problems.Hardware problems include mis-wired communication cabling and faultycommunication interfaces.Software (protocol) related issues arise when the master application tries to access nonexistentnodes or use invalid function codes, address non-existent memory locations inthe slaves, or specify illegal data format types, which obviously the slave devices do notunderstand. These issues will be dealt with in detail later under the Modbus Plus protocol,as these issues are common to both the traditional Modbus protocol and the Modbus Plusprotocol. They are summarized under software related problems.Note that these issues are also applicable to the latest Modbus/TCP protocol, which isdiscussed in appendix D.7.4.2 Description of tools usedIn order to troubleshoot the problems listed above, you would require the use of a fewtools, both hardware and software, to locate the errors. The most important tool of allwould always be the instruction manuals of various components involved.The hardware tools that may be required include EIA-232 breakout boxes, EIA-232 toEIA-485 converters, continuity testers, voltmeters, screwdrivers, pliers, crimping tools,spare cabling tools and other similar tools and tackle. These would generally be used toensure that the cabling and terminations are proper and have been installed as per therecommended procedures detailed in the instruction manuals.On the software tools front, you would need some form of protocol analyzer that is in aposition to eavesdrop on the communications link between the master and slave modules.These could be either a dedicated hardware protocol analyzer, that is very expensive, orsoftware-based protocol analyzer that could reside on a computer and act as theeavesdropping node on the network.Obviously, this second option is more economical and also requires the relevanthardware component support in order to connect to the network.Figure 7.12Screenshot of the protocol analysis tool