Modbus Analysis with Prosoft MVI56-MCM

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This is a step-by-step writeup of how to do some automated Modbus protocol analysis on data passing through a Prosoft MVI56-MCM module using only Windows Hyperterminal, a text editor, and the excellent free Modbus Parser from Chipkin Automation Systems (

This technique could be handy for debugging or monitoring a Modbus data stream during installation and commissioning of a MVI56-MCM, or for checking for a certain Modbus transaction in a stream of data. It's certainly a lot easier than trying to decode Modbus manually.

This technote works with the "classic" MVI56-MCM modules that Prosoft has been selling for about the past 10 years. It should also work with any of the MVI-series modules for the SLC, PLC-5, or FLEX platforms that run the same basic firmware.

The newest MVI56E modules have a web-based monitoring mechanism instead of a diagnostic serial port and I don't know if they have a Data Monitor function.

I still prefer the full-power Frontline Test Equipment ( NetDecoder suite of software, but that's because I do automation communication analysis for a living. For casual Modbus work, the Chipkin tool is a great labor reducer, and the amount of labor can be further reduced using Hyperterminal and a text editor such as UltraEdit.