Hello,

I am new to the programming PLC field.

I am using TwinCat 2 (very similar to Codesys).

I am trying to read the floating variable via MODBUS RTU.
The read variables are in a word array. How can i convert 2 word into float/real?
Moreover I need to swap order of these words.

I know that I can do it via memory allocations but there is too many variables.

Thank for help.

Hi,

Maybe you should try to search this forum for "word to real".
There are more then several postings on this specific problem.

Good luck ;-)

https://forge.codesys.com/forge/talk/search/?q=word+to+real

I do this quite a bit. I use a union, and the ROL function depending on endianness.

The union. I called it 'TypeCast' because that was the equivalent function in LabVIEW.

TYPE TypeCastDWReal :
UNION
    dw  :   DWORD;
    rl  :   REAL;
END_UNION
END_TYPE

Then I have two function that use the union depending on the direction you want the conversion done.

FUNCTION TCRealSwapped : REAL
VAR_INPUT
    dw  :   DWORD;
END_VAR
VAR
    MyUnion :   TypeCastDWReal;
END_VAR

MyUnion.dw := ROL(dw,16);       //  Word swap dword input because it's big endian on ModbusTCP
TCRealSwapped := MyUnion.rl;    //  Function (via union) returns real

FUNCTION TCDWordSwapped : DWORD
VAR_INPUT
    rl  :   REAL;
END_VAR
VAR
    MyUnion :   TypeCastDWReal;
END_VAR

MyUnion.rl  :=  rl;                         //  Input to function is REAL
TCDWordSwapped  := ROL(MyUnion.dw,16);      //  Function returns DWORD, needs word swap for ModbusTCP register space

Then I step through the modbus data array via pointer and byte offset, and pass the dwords/reals into the function block.

This is reading a real input to my server.

pDword  :=  ADR(awEMS) + 12;    dsEMS.i.rRealPowerDemandkW          :=  TCRealSwapped(pDword^); 

This is writing real into the data array outbound on my client...

pDWord  :=  ADR(awEMS) + 2014;  pDWord^ :=  TCDWordSwapped(dsEMS.q.rRatedACVoltage);