RS UNI Modbus RTU

PLCnext Engineer
1

Hello,
I am currently trying to communicate in modbus RTU (rs485) with a Rs uni module (2688666).
I can see that I correctly send my requests but I get nothing in return.

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I also see that none of the two blocks have error code.
Normally I should get data in my rxdata table right ?
I’d also like to add that I’m no expert on the subject !
If it is possible to help me please !
Thank you
Aymeric

2

Hello!
First of all, do you bind inputdata and outputdata variables to the rs uni module configuration? You may see led blink on rx tx module operation.
Also, does settings of module itself are identical with modbus device network settings?
br

3

Hello !
Yes I do :

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And yes I setted the same parameters on my device and on my rs uni module
thanks

4

Ok.
It is not nessesary to bind RxCHAR* or other similar variables in this case.
Also, please set RS uni mode as TRANSPARENT

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And set MODE input of Modbus master FB to FALSE.
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BR

5

Thanks it worked !
But just a question, why do we have to set transparent protocol while we use RTU protocol ?
Best regards

6

There is some issues occurs in case of this combination of parameters. I don`t know exactly what is going on, but this is known bug feature for a while already…
Probably some one of authorized tech team can tell more detailed.
BR

7

As far as I am aware, this is not a bug.
In a PLCnext Engineer project, the Modbus Client (Master) functionality can be implemented in one of two ways with an RS UNI:

  1. In the RS UNI module itself, by selecting the Modbus protocol option in the module configuration. In that case, you shouldn’t use the Modbus library in PLCnext Engineer, but instead should follow the instructions in the documentation for the RS UNI module.
  2. Using the Modbus function block library. In this case, the Transparent protocol option should be selected in the module configuration, because the Modbus protocol is implemented in the function block library.

When the RS UNI is used with PLCs from other vendors, via an Axioline Bus Coupler, then the second option is obviously not possible, but the first option is still possible.

8

Thanks a lot for all this information !
Best regards,
Aymeric