Modbus Communication using Special Message Framing AC500 via Emerson Floboss

Hello,

I am using Modbus RTU communication at AC500 PLC via Emerson Floboss 107 flowcomputer.

There are some special historical data registers that I need to read. It doesn't work like standart communication protocol.

Instead of "Number of data", "Number of index should" be send to device. ( Number of data is already defined at the device).

Message framing is like;

01 03 00 B6 00 0C XX XX (01 as adress, 03 function code, 00 B6 as hex of register 182, polling for index 12)

With this way, I can use NB ( Number of data ) input of COM_MOD_MAST block. But I get error of "8215" Invalid length.

I can see reply of slave using Modbus test software, and this's how it works at slave side.

Slave actually sends data that I need, but COM_MOD_MAST block does not write data to my variables because of this error.

How can I overcome this error? Is there a way to make COM_MOD_MAST block to ignore this error and write received data to my variables?

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yavuzz Rank: 528

Asked 3 years ago

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Answers

Hello!

Are you using the exact number at NB as the it send back from the slave?

Maybe you have to use ASCII and COM_REC/COM_SEND and define this telegram as it is asked for by this Emerson component...

I know it is not an answer but a suggestion.

Gustav

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GustavA Rank: 52

Answered 3 years ago

Are you using the exact number at NB as the it send back from the slave?

If I use exact number, I can see data. But that exact number is an index. I can't get another indexes, because data number at NB and index does not match.

I tried to generate Modbus telegrams using Syscomlib, but couldn't manage to do it. ASCII and COM_REC/COM_SEND sounds better. Will try

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