CODESYS - the IEC 61131-3 automation software

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PostPosted: Tue Jul 25, 2017 6:30 pm 

Joined: Fri Feb 24, 2017 10:59 pm
Posts: 2

I have some analog sensor threshold constants defined in my code. Even though the sensor value is delivered as a 16-bit integer, I normally like to define the thresholds using REAL data types, with SI units (example: 112.5, units of °F). This makes the code easier to read and modify.

However, in the code, I need the threshold converted to the 16-bit integer for the algorithm to use.

In C code, this was a pretty simple thing to do, as I could use a #define macro to perform the conversion, and as long as my coding style ensured that the value was "const/read-only", it would optimize and implicitly convert it to the 16-bit int (no need to perform operations with REAL data types at run-time, or even have a REAL data type stored in memory).

Is there something similar to this in structured text?

PostPosted: Sun Sep 24, 2017 6:26 pm 
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Joined: Fri Sep 02, 2011 8:02 pm
Posts: 302
There is not an equivalent to the define of C language. The closest you can come is to define the conversion as a constant first (if you can) and then define your 16 bit constants as a REAL x conversion.

For example:

   TRIP_TEMP : INT := REAL_TO_INT((111.5 - 32) * 5 / 9);

But I would suggest just defining the real and making the conversion during code execution using a function. Unless you are running a low power embedded system, you wouldn't notice a problem. And even in an embedded system, you probably wouldn't notice anything as most have floating point chips anyway.

In my machine programs, I mostly convert all my variables to reals first! Fieldbus values - convert to reals every PLC scan... Then I can limit my overflow/underflow errors to a small area. Plus it's much easier to debug when I see DriveTemp = 57.3 (C) instead of 573.

Famous quote: "The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming." Donald Knuth

Scott Cunningham
KEB America, Inc.

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