Modbus signal testing

This post is not related to any automotive diagnostics but I believe it show how versatile the picoscope is.
I work at a gas distribution facility where we have some automated actuators for our ESD system. Recently we started experiencing some problems with this system. When all 17 actuators are powered communication is lost from our PLC system. Checks was made by the supplier of these actuators and also our SCADA team. The problem couldn’t be found since communication was seen at the respective actuators and there was no break in comm. lines from the field to panel.
I saw this as an opportunity to use the picoscope out of curiosity and didn’t expect to find anything since the 4425 was not made for modbus testing. However it worked and also the automotive software can decode modbus rtu 485 protocol.
Out of the 17 field components, 6 caused a disturbance on the network. The following is some pics of what was found.

4 channels connected to two comm lines.


Known good packet of modbus data







After realizing what was happening. We then understood why we keep loosing communication. This data will now be sent to the supplier and we await a response.
If wasn’t for my picoscope this issue will be long from figuring out. This testing took approx. an hour and a half yesterday. And I got a full 12hr pay for using my scope, not too bad for a novice scope user. And of coarse further testing will have to be done so maybe after all this my scope will be paid off for.

I hope who reads this find it interesting.



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This is cool stuff. Thanks for posting!

I have questions. :lol:

Is the normal Modbus voltage communication 0-5V? What is the normal 'neutral' voltage on the network when no messages are present?

Is this network fault tolerant? Will is work in single wire mode, or are both wires needed for communication?

Are you paid on some kind of flat rate system? Because 12 hours for 90 minutes work is pretty good. :woohoo: Is scope testing atypical in your industry?

hi sorry for such a late response, was on vacation.
Neutral voltage when no devices is powered is zero. As more devices are powered, on the network the neutral voltage will increase. So it may be 6v or 8v all depends on how much devices are there. The modbus signal voltage is approx 3v, therefore if the neutral voltage is 6v the comm. signal will be at 9v.
Both wires are needed master/slave.

Scoping this to find an issue was my decision, not part of my job but having this issue present it affects the daily operation of the facility where I work. this loop controls 17 actuated valves on a ESD system ( Emergency Shut Down) Each actuator costs like 2M Usd. Having it inoperable is a big problem. The system was checked by the supplier and a scada analyst they couldn't find the issue. Thats why i decided to scope the comm lines. www.scannerdanner.com/forum/welcome-mat/...hi-new-guy-here.html
Link to my introduction.
I work shift, so i came out on an off day, therefore I claimed 12hrs double for scoping this. In my opinion i did them a big favor because at this point everyone was scratching there head.

Hope I answered your questions.