Some of the Modbus configurations only accept WORD type variables. If you need to convert a REAL value to 2 WORDS, you can use UNION data type to achieve this easily. Different data types in the UNION share the same memory space. 

Create a DUT called ST_REAL_TO_aWORD:

TYPE ST_REAL_TO_aWORD:
UNION
	fREAL : REAL;
	aWord : ARRAY[1..2] OF WORD;
END_UNION
END_TYPE

Declare the variables in your program: 

fRealValue  	: REAL := 0.81;	
stData		: ST_REAL_TO_aWORD;

You will see that the REAL value is converted to 2 WORDS automatically in the UNION type. 

You can also apply this method to other data conversions (BYTE to REAL, INT to BITS, WORD to BYTE etc...) as well.