Carry Flag - Allen-Bradley ControlLogix 5570 Reference Manual

Chapter 6 Instruction Execution
Language Affected
Ladder Logic (RLL) Yes
Structured Text (ST) Yes
Function Blocks (FBD) No
Sequential Function Chart (SFC) Yes
(1) Only affects embedded Structured Text.
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Carry Flag

Only the ADD and SUB instructions that specify integer operands can affect
the carry bit. This lets you perform chain calculations to support larger
(unsupported) data types (Add low, Add_with_carry high).
The carry flag is limited to only those operations that are relevant. This
simplifies its use and that of math status flags in general.
(1)
This implementation affects all instructions that can set math status flags.
Use of the ADD and SUB operators has no effect on the
carry bit.
Only ADD and SUB instructions with integer operands
can affect the Carry flag.
Mitigation
Look for references to s:c in the user project and verify that the logic functions
as intended.
Rockwell Automation Publication 1756-RM100F-EN-P - October 2018
5580/5380 Controllers 5570/5370 Controllers
Many math instructions can set the Carry bit.