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ModBus address access limitation

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Hello to all.

I am working on a project in which I will only get PLC data to an external control system, we will do it in MODBUS.

Everything works correctly, but I find two problems:

The addressing mapping is 1: 1 accessible from MODBUS, so they could read any record.

The same is reproduced in writing, so that accidentally (or not) they could overwrite important values of my program.

Could we limit the reading to a certain number of records?
Could we limit the writing?

Thanks in advance!

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Hi HexMan,

This option exist for the enhanced models – Please make sure to use the latest OS:

1 Set in SI165 the start address of MB the user will be able to access

2. Set in SI166 the end address of MB the user will be able to access

3. Set in SI167 the start address of MI the user will be able to access

4. Set in SI168 the end address of MI the user will be able to access

5. Set SB305 to activate the MODBUS limit.


For example if you will set the following:

SI165 = 1000

SI166 = 1999

SI167 = 1000

SI168 = 1999

SB305 = 1


The MODBUS master will be able to access only to MB1000 – MB1999 and MI1000 – MI1999.


If anyway one tries to access the restricted operands SL46 will increment.


You have to arrange MIs and MBs in sequence.


There is ni separate limitation for read and write.



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Since none of what has been presented here is documented anywhere, including Visilogic, is there a document that can be made available for these ranges?  It seems that MODBUS access security is going to become very important with the new cybersecurity laws being passed in California and elsewhere and it would be nice to find out if there are any undocumented features or other methods to implement cybersecurity in our PLCs.  I noticed that the information presented by @AlexUT is not documented in Visilogic 9.8.80.

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Until we will update RN/Help, you can use next to check if MODBUS limitation is working or not in your version:

As soon as you can write into stated SI/SB, MODBUS limitation should work.

*System registers are write protected, if not used for additional functions.

At this time only MIs and MBs MODBUS limitation works.

We added request to wish loist for feature implementation.



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