floating point add fct dont work

Description:
Generally speaking, it holds that REAL numbers are specified and calculated with an accuracy of 6 decimal places. With the calculations you should also note that this accuracy holds for every single step of the calculation.

When adding and subtracting, the exponents are aligned, i.e. the basis and the exponents are the same for adding and subtracting, only the mantissas are added.

Example 1:

To be calculated:

y = a + b - c;

z =1 / y;

with

a = 100 000 000 as REAL value = 1.000000*10⁸

b = 1 as REAL value = 1.000000*10⁰

c = 100 000 000 as REAL value = 1.000000*10⁸

• In the first step of the calculation (a + b) is determined:
  (a + b) = 1.000000*10⁸ + 1.000000*10⁰ after alignment of the exponents you get
  (a + b) = 1.000000*10⁸ + 0.00000001*10⁸. The last two places are truncated, because they can no longer be displayed (accuracy of 6 decimal places).
• So a zero is added.
• In the second step of the calculation ((a + b)-c), c is subtracted from the result of the previous step of the calculation:
  ((a + b)-c) = 1.000000*10⁸ - 1.000000*10⁸ = 0.000000e⁰
• If z is now determined, then you attempt to divide by zero!

The correct solution is obtained in this case by converting the calculation.

y = a - c + b;

z = 1 / y;

Since here, after the calculation (a - c), the result 0.000000e⁰ is obtained, in the second step of the calculation ((a -c) + b), the addition of the REAL number leads to the correct result.

y = 0.000000*10⁰ + 1.00000*10⁰ = 1.000000*10⁰

Therefore, before programming a calculation, you should always check how the calculation can best be made.
More information is also available in the STEP 7 Online Help under the Topic of "Real: data type".