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Modbus Sniffer Software
Sniffer SW for Modbus via radio with communication problems.

I have two Modbus systems (scada-master and slave-logger) connected via radio. I have the ability to use two types of radio-modem with RS232 port. One works at 2.4GHz and the other at 868MHz.

With the first couple of radio couple 2.4GHz everything works. I use Chipkin to simulate the scada, request are sent for registers 16 (40001 up to 40016) and the slave responds fine.

With the couple of radio-modem 868 MHz, the request arrives at the remote slave (I can see the RX LED that lights) but the logger slave does not respond. So I want to see if the couple of 868 MHz change the string that reaches the remote slave.

Please anyone can suggest a SW that can occur if the request comes changed? Like a SW that sniffer and converts the string.

To do this I have set up another PC with the serial port with only GND and RX wire. For each scada request, I can see coming strings that look the same, whether the radio is at 2.4GHz, both in the 868MHz. It is possible to parse strings to see if they are different. Of course I make these test in a room, in the laboratory. The different frequency and short distance do not make difference and problems. Also because with other sw and other protocol all systems works,with both types of radio!

Many thanks in advance and sorry for my English.

I have an RS232 sniffer Windows Console application with a T cable that diode auctioneers Tx and RX into one serial port RX.

email for further details.

>I have an RS232 sniffer Windows Console application with a
>T cable that diode auctioneers Tx and RX into one serial
>port RX.

Thanks Harvard for providing me with this diagram (in Dec. 2013) to post:

It is linked here:

Could I also have more information on your console application? I'm not asking for something to post (but might if you request it).


Thanks for your interest in my post.

My though the question is to find an analyzer of strings. a converter, in ASCII for example, the strings that are sniffed, to understand if the strings come right, or that displays me (without having generated the request for Modbus data) ithe registers and values required by expires.

I tried with REALTERM, but not has the MODBUS protocol and looking string in other formats, sound the same or illegible (of course for my lack of knowledge of other codes such as binary hex UIT18 etc. etc.)

Many thanks


The link above has a PDF linked in the third paragraph and I'd suggest reading Part 3 chapter 7 "Data and Data Types" to get more familiar with such topics.


Dear Paul,

thanks again, it is clear what I mean? my problem?

I always in doubt for not explain well, even for my bad English.


Yes, Sandro, your problem is clear, but my last message might not have been that helpful. The link in my last message was good for learning programming, but not very good explaining hex and binary:

Here is a better explanation of hex and binary:


I did want to show you something else Sandro that might be of help. Here is the Control Terminal (free and open source) program screen shot with a simple Modbus transaction:

I worked (part-time) for 16 years on a lot of communications software for equipment made by this company:

The log format on the bottom of CtrlTerm is what I've used for years. It has a (partial) time stamp and also "T" or "R" characters that show if the data was transmit or receive.

On the right side are hexadecimal for every byte, and ASCII value are shown on the left (if it is printable).

This is a handy way to display strings and byte data. Of course, if byte data is combined for 16-bit or 32-bit values you'll have to figure that out yourself.

Does all that make sense?


Dear Paul,

great, you made the center!

I'm just working with a CSI logger, a CR1000.

If you have taken to heart my problem and want more information, I wrote a post about this in the CSI forum (my name is smile). I also made some comments to an article in the CSI blog of Paul Smart "One Helpful Method to Diagnose a Modbus Communication problem ".

Since that forum is dedicated to their logger, I described the specific question.

Thanks again.
I will try the sw you suggested.



> I'm just working with a CSI logger, a CR1000.

Wow, I had no idea!

This might be interesting to you:


Dear Paul,

I work with CSI since 1985 and rarely happen to me like puzzles! I have received an answer from the CSI forum, and I think JDavis right. The fact that at 2.4GHz you can have a band, much wider, makes sure that there are no changes in the data packet sent.

Thank you very much for your cooperation.
Dear Paul will be my guide in the next puzzle!


Try this solution to sniff your Modbus data -