Allen-Bradley LOGIX 5000 Reference Manual page 526

Chapter 10 Structured Text Programming
The FOR_DO loop executes a specific number of times.
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Important: Do not iterate within the loop too many times in a single scan.
The controller does not execute other statements in the routine until it completes the loop.
A major fault occurs when completing the loop takes longer than the watchdog timer for the
task.
Consider using a different construct, such as IF_THEN.
Description
The syntax is described in the table.
This diagrams illustrates how a FOR_DO loop executes, and how an EXIT
statement leaves the loop early.
Affects Math Status Flags
No
Major/Minor Faults
A major fault will occur if
The construct loops too long.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
To stop the loop before the count reaches the last value, use an EXIT
statement.
Fault type Fault code
6 1