ABB AC 800M, Library Object Style Manual, Page: 107 / 120

Section 6 Program Code Issues Overflow handling
3BSE042835-600 107
Overflow handling
The object must have a predictable behavior when calculating output parameter
values. Situations like divide by zero and sqrt() of negative numbers shall be
prevented or handled in the code.
Example:
Create a function Y := (1/T)*G, where T and G are parameters to a function block.
Solution:
When T is too small, the calculation generates an overflow in the arithmetical
processing unit. To avoid this, the parameter T must be checked. As T is assumed to
be a time, only positive and not too small values are allowed.
Code:
IF T < Ts THEN
Y := G / Ts;
ELSE
Y := G / T;
END_IF;
Where Ts is the selected sample time for the running task.
The requirement is that the library object shall not give an overflow output value if
none of the input parameters have an overflow value.
SIL Mark Restrictions
Objects need to be restricted marked if any of the below is true:
• The object calls the functions that are not fully executed in the SM81x safety
module.
• The output data from an unsafe source is not secured with a safety layer.
This is only relevant for 800xA internal libraries.
To obtain the main functionality of the object in the SIL environment, the whole
object must be SIL Restricted.
If the object is set to SIL1-2 or SIL3, but the parameters are Non-SIL, then it is
possible to obtain partial functionality in the SIL environment.