How to sniff/check Modbus Frame

I would like to get data from my energy meter via Modbus RTU. I checked connection directly from computer using QModMaster and it works fine. Modbus implementation at this energy meter I think is very poor. In documentation is only four things described to use and without registers numbers, but is described... frame. E.g. to check energy consumption I have to send question, which looks exacly: 02 03 01 1e 00 02 + append CRC sum. I can generate this question in QModMaster with parameters:
UnitID: 2
Function: 3
register: 287 (but 01 1e is 286? )
quantity: 2
When I am putting this same parameters through Node-red, I got a timeout.
So have you any simple trick to see how the question frame looks?
Data taransmission parameters are right (9600, 8E1) like in QModMaster.

Go here: Online Modbus RTU Parser & Modbus TCP Parser


So Steve was able to show you that your assumptions off of the frame were correct. That leaves something about the setup.

What nodes are you using to work the MODBUS setup in Node-Red and how are they configured? Are there any other options you can configure in QModMaster that aren't listed in your post? Just trying to figure out how your implementation is setup.

My first impression I see is that you're using a RS-485 converter or something similar to have a baud rate set in your code. Some Node-Red nodes don't like to work that way unless you set them up very carefully. My guess is that you've already double checked that so it's not likely an issue. And we know the hardware works fine because you can get an alternate program to utilize it correctly.

That leads me to my second thought and some of the questions above. From the looks of things, QMM is doing something NR is not. That could be based off the nodes chosen. It could also be based off of the node configuration. Generally, both the nodes and the setup are very simple and robust. But there's the question...

If you're looking at actually capturing the query being sent to your MODBUS unit, which is what I think you're really asking, you'll want to find a program for serial ports like Wireshark is to networks. A cursory search for "serial port sniffer" on Google brought up this option. And there are probably dozens more besides. There are always the hardware options, but those cost money and we're trying to do this without spending if possible. Utilizing a software option should show you the flow going to/from your MODBUS unit and the serial converter.

Without seeing your nodes themselves and trying not to duplicate Steve's work, that's about all I can add. Good luck and let us know if you have anything else come up or if you can post your flow.

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.