Need help conf to read input from user and use in expression

[smarthacker]

I am new to fhem.
I have device that transmits the power along with the serial number.
While indicating on UI I need to Subtract the value from specific formula.

Is there in any way in conf to define variable to store $serial_number which user can input.
Which I can use in expression eg.

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attr MYDEVICE obj-h123-expr $val - $serial_number


I tried something like(may be a wrong as well) but its not working

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define MYDEVICE  ModbusAttr 1 1 192.168.101.1:503 TCP
attr MYDEVICE ReadingsVal($serial,"serial_number")
attr MYDEVICE obj-h123-expr $val - $serial
set MYDEVICE obj-h123-serial_number 1234
attr MYDEVICE obj-h123-poll 1
attr MYDEVICE obj-h123-reading power

Can someone help/advice ?
Let me know if I have to write new module it self for this.

Thanks in advance.

[amenomade]

Where do $val and $serial_number come from?

The right way to calculate new values depending on Readings of some Fhem Devices ist using userReadings https://fhem.de/commandref_DE.html#readingFnAttributes

[amenomade]

And to get input from user, you can for example define a dummy, and then in th UI you have the possibility to user the set button

[smarthacker]

Where do $val and $serial_number come from?

The right way to calculate new values depending on Readings of some Fhem Devices ist using userReadings https://fhem.de/commandref_DE.html#readingFnAttributes

I just tried this to check if something like this works

[smarthacker]

And to get input from user, you can for example define a dummy, and then in th UI you have the possibility to user the set button

Thanks for the pointers I will try this now and let you know.
Will it be possible to use this value while evaluating the expression?

[amenomade]

If you define a dummy

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define serialid dummy
and then through the UI you do

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set serialid 45616845498
then you can use this value in a userReading with {Value("serialid")}

[Wzut]

@smarthacker, I cant believe that your Modbus device has two different values in one single register !
What's in the Modbus documentation for your device ?   

[smarthacker]

@wzut not it do not have 2 different value in a single register. This is impossible I have 2 different values in 2 registers which I need to combine
Simply 32bit = 16bit (reg1) + 16bit (reg2)

[smarthacker]

If you define a dummy

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define serialid dummy
and then through the UI you do

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set serialid 45616845498
then you can use this value in a userReading with {Value("serialid")}

Thank you so much for the help. It did work but I need to exclude {}.

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Value("serial")