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Modbus TCP Error: Suddenly Invalid data?




We are trying to monitorize some sensors that retrieve the temperature from a room via Modbus TCP and already have mapped all directions and port so we can connect succesfully. The thing is, that we are able to retrieve the information correctly (the temperature is shown correctly with 16 bit integer although we have to divide by 10 so it shows the real value, otherwise, the value is exponentially high and makes no sense) and everything looks like its going fine and without issues, we connect succesfully, and retrieve data as we wish.

The problem is, that at some point, the retrieved data is unknown and it gets to the Warning status, and later on, it will go to Down status and the error shown is: Request reading 1 holding registers starting at 800 failed: Invalid data. This, makes no sense, because if this would be the case, it wouldnt get the data in the first place am I right? Sometimes it will reconnect correctly but more often than not, it will go to the Down status.

Also, if I just pause it and resume it again, it will usually recieve the data correctly again so I´m kinda lost on what can be happening or what is the problem that I´m facing. Any advice is helpful.

Thanks for your attention and greetings,


invalid-data modbus-tcp prtg

Created on Oct 4, 2021 8:00:44 AM

2 Replies



Hello Diego,

Thank you for your KB-post.

Which version of PRTG are you running?

Please make sure that your PRTG installation is on the latest version (, as we introduced some improvements and big fixes for Modbus sensors in that version.

Created on Oct 8, 2021 12:15:47 PM by  Isidora Jeremic [Paessler Support]



I´m running the latest version or PRTG, but I figured out the problem, it´s just that we had a program that also requested info and it was too frequent, so it was collapsing with the PRTG, but everything is alright now, just add more time for the querys on our program and it works fine.

Created on Oct 11, 2021 6:55:30 AM

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