floating point - PLC Structured text, convert decimal to real(float). Not getting the value I expect. (IEEE-754) -


i have hardware communicating plc on profibus dp, sends me 4 bytes of data in hex "44 79 ff ff" , in plc program have declared byte array input data. problem in plc data receive in byte array "66 121 255 255" decimal value of hexdecimal, goal convert value real , when i'm not getting value expected.

i have created dword(4bytesdata) insert bytes into. before insert 4 bytes 4bytesdata bytes are: in1 = 68 , in2 = 121, in3 = 255 & in4 = 255

4bytesdata := (shl(shl(shl(byte_to_dword(in1), 8) or byte_to_dword(in2), 8) or     byte_to_dword(in3), 8) or in4);  realvalue := dword_to_real(4bytesdata);

where in1, in2, in3 & in4 byte 0-3.

the value i'm getting is; 4bytesdata = 1148846079 & realvalue = 1.148846e+009

the value i'm expecting realvalue = 9.9999993896484375e2

if use website ( ieee754 analyzer/converter ) , converting hex value (4479ffff) i'm getting value want to, , if insert decimal value (1148846079) i'm getting same value receive in plc.

i hope understand problem, many in advance.

you've figured out, seems don't realize it. conversion dword_to_real taking integer (hex) value stored in 4bytesdata , converting ieee754 real format.

this not want do. 4479ffff already in ieee754 real format - you're taking value, interpreting real dword, , converting dword value real. short answer don't need conversion - 4bytesdata in correct format.

edit

follow on comments :

enter image description here

here functionblock2 delclares rin(real), rout(real) , sets rout:=rin;. gets around forced typecasting of st. nothing changing data stored in d2000 here - it's same binary data. top function block storing memory , bottom 1 reading memory. difference top rung interpreting dword (for display purposes) , bottom rung interpreting real.

edit

i've been reading beckhoff manual. seems may have other options. try perhaps declaring dword , real @ same memory location. if not, perhaps pointers let it (not sure if type restrictions apply pointers beckhoff?)


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