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Modbus TCP communication with UMAC


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I have an application about the Modbus TCP, There are two sub stations which is taken as client, the both sub station have some input/output module separated , and take UMAC which active socket is 2 as Modbus Server .

I set I67 = $10700, so the start address of client Modbus Buffer is $10780.

I have some puzzle about addressing for each client.

when addressing, if the both sub stations occupy the addresses starting with word 0, how to distinguish their address? or how to avoid overlap address between two sub station?

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I knew that the both sub station need to be set different IP address.

the question is that how to distinguish the start Modbus address for each station?

From the data structure, the start of client/server Modbus buffer is always from 0x80 + I67,

so if the both client occupy the start buffer(0x80 + I67), does it address conflict?


for example, if I set M1128->X:$10780,0,1 , then I read M1128 in PLC, which sub client I read?


do I describe it clearly?

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There is no conflict if each Client on the PMAC uses different Modbus addresses in the User Buffer.


However, the same addresses can be written to. It's just that what one writes will get overwritten by the other. I think it might be necessary to hand shake so not reading or writing at the same time. That's how I did it in some test programs.

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I reread your initial post, and now I don't understand the setup. It sounds like there are two Clients and maybe two Servers, and the UMAC is one of the Servers. If that is the case, I can't speak for the other Clients.


If PMAC is a Client to multiple Servers which have the same Modbus addresses, there is no conflicting or overlapping because each Server will have a unique IP address, and those addresses will be mapped to unique addresses in PMAC. For example, see the attached "ExampleSetupScreen" to see how it's done in the Modbus Setup window, copied from the attached "MODBUS TEST SETUP EXAMPLE" document.



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  • 2 months later...

I want to confirm that the Modbus Reference and PMAC Reference are both offset value or actual address? 

I want to read multiple register from Festo Module using modbus, the Modbus reference address is from 45392, and the Pmac reference is 60($10780 + $3C = $107BC), 






the detail setting is below,  actually M2120 to N2137 can't read the status of Festo Input module. 

and I also try to set Modbus reference is 5392, still can't work.

what's wrong ?



2022-03-06 16_36_11-Untitled - CPX-FMT - [ONLINE TCPIP].png

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