7669 questions

9105 answers


12097 members

0 votes
219 views 4 comments


I have a brand new RUT955 with last firmware.

If i read modbus registers, with modbus TCP/IP, with my PLC, it seems to work fine. But if I use modscan32, every time I connect it read values 1 time and after that it stops with a "connection terminated" error.

If I check the "keep persistent connection" flag, It works with both devices, but only for a minute. After that it has to be restarted.

How can it be solved?

1 Answer

0 votes

Hi Francesco.

Sorry for late response.

If I understood correctly, everything works correctly when reading registers from one device (PLC). Could you test with other testing software (like Modbus Pool) or do you have another device that supports Modbus? Notify me if the issue persists.

Best regards.


Hi Igor,

Modbus Poll keeps the connection alive and shows data, but every reading produces an error.
Funny thing: one reading error, one writing error, one reading error, one writing error, ... (even if I'm not writing).

Bonus fact: when selecting "keep connection alive" in the latest teltonika firmware, modbus32 stays alive. For around a minute :| then connection falls. PLC instead cannot communicate.

Bonus fact 2: with modbus tcp slave activated, after startup the RUT955 does not activate the 4G connection! It takes a IP from the mobile network but "no connection" is shown on the connections graph. Only after writing "1" in HR204 (mobile data ON) mobile data start to flow. This happens at every startup.


I have connected my rut955 as a slave. As a master, I use another router and software ComTest Pro. So far I have not been able to reproduce what you described. The connection has been stable for a long time. Perhaps you could test the rut with a different device.


I already tested the router with 3 different modbus masters.

Beside this, modscan32 is a "de facto" standard in modbus tech so it MUST work with this software.

Further beside that, even with NO modbus master software/device, after startup 4G connection does not start if modbus slave function is enabled. So it is not a problem of which modbus master I use: this problem exists also without master attached.

Very sad that Teltonika does not have a "bug report" system.


Please, send me troubleshoot file via PM.

You can download it here System->Administration->Troubleshoot