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Modbus to Siemens 353 Controller


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Customer has a Siemens 353 Controller and needs to communicate to it via Modbus ( we are using Serial RTU)

Unistream  is the Master, 353 is the slave.  Communications is working properly

We can read bits from the 353  like the push buttons, after adjusting the addressing by 1   ( Siemens 342  = Unitronics 341)

We are having trouble with the Integer registers, they are coming back blank.

Siemens   40061 - 40066   are basic clock data and we cannot read that, let alone the data that we need.


Anyone with experience with this controller?  a few more clues?




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Hi Dan,


Are you using Real numbers? We have similar issue using Real numbers. 


A real is a 32 bit register. In our case, the UniStream and Siemens controllers did not interpret the 4 bytes in the same order. (i.e. 8e1 is interpreted as 81e0, or the like) This results in different values. 


In the UniLogic software, there is a function block that will swap the bytes. This worked; however, if there are not sequenced properly, it will result in "flickering" of the value between 2 values. 


A similar issue can occur with Integer values as well. 


Please feel free to email us at support@unitronics.com for further information. 



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