Safety Manager Safety Manual - Tuv-fs.com

3 – Safety Manager fault detection and response
Calculation errors
Caution
Safety Manager stops if a calculation error occurs.
Calculation errors may occur in the application program.
Calculation errors occur if:
• The calculated value of an analog output is outside the specified range.
• The square root of a negative number is taken.
• A logarithm function is loaded with a negative value or zero
• A divide-by-zero occurs.
• An overflow occurs during a calculation.
• The value for a counter is outside the specified range.
Calculation errors reflect an incorrect design of the application program for the
intended function. Once a calculation error occurs for a specific process point, a
correct result of successive calculations based on this point cannot be guaranteed.
Guidelines on how to avoid calculation errors in the Safety Manager application
are presented below.
Preventing calculation errors
Calculation errors can be prevented as follows:
• Overall process design.
• Inclusion of Safety Manager diagnostic data.
• Validation of signals in the Functional Logic Diagrams (FLDs).
• Exception handling during the actual calculation.
Prevention by design
In line with good software engineering practice, as promoted by IEC 61508,
calculation errors should be avoided by design. This means that an application
should be designed in such a way that the operands of a symbol in the FLDs can
never get an invalid value. The design approach starts with making sure that input
values as obtained from the process remain within a predefined range. This
approach ensures that the derived values are also valid for successive operations.
34 Release 131, Issue 4.2