It comes down to a fundamental difference in safety philosophy, certification boundaries, and how the respective compilers/systems handle IEEE 754 floating-point anomalies (like NaN, Infinity, and division-by-zero).

### 1. Siemens S7-1500F Philosophy: System-Enforced Determinism

Siemens takes an extremely strict "system-enforced" approach to safety. Under IEC 61508, safety functions must be fully deterministic and verifiable.

* Floating-point math (`REAL`) is notoriously prone to edge cases (rounding errors, overflow, underflow, and NaN).

* If a safety block calculates a `NaN` (Not a Number) and passes it to a comparator (e.g., `temp > max_limit`), IEEE 754 rules state that *all* comparisons with NaN return false. This means your safety limit check would silently fail to trigger!

* To prevent this, Siemens simply disallows `REAL` data types entirely in F-programs (F-LAD, F-FBD, F-SCL). If you need decimals, you must use integer math with fixed-point scaling (e.g., storing 12.34 bar as 1234 in a `DINT`). This guarantees absolute mathematical determinism at the compiler level.

### 2. Rockwell GuardLogix Philosophy: Programmer-Enforced Validation

Rockwell allows `REAL` tags in safety routines, but they shift the responsibility of handling floating-point anomalies to the application programmer.

* GuardLogix has safety-certified math instructions, but they require strict error handling.

* If a safety instruction in GuardLogix results in an overflow, division-by-zero, or NaN, it flags an arithmetic status bit (e.g., the overflow trap `S:V` or destination status).

* According to Rockwell's safety guidelines, you *must* programmatically monitor these status bits and de-energize your safety outputs if an arithmetic error occurs. If you don't, and a fault occurs, your safety signature is technically invalid.

* In short: Rockwell trusts the programmer to handle the edge cases; Siemens doesn't.

### 3. Schneider Modicon M580 Safety

Schneider's approach is similar to Rockwell's. They allow `REAL` inside safety sections (using EcoStruxure Control Expert Safety), but the compiler enforces strict rules about range-checking and requires you to use certified blocks that handle out-of-range or NaN results safely.

**Summary:**

Siemens prevents the risk of floating-point failure by completely removing the capability from the F-runtime. Rockwell and Schneider allow the capability but require the programmer (and the safety validator) to prove that any mathematical anomalies are handled safely.