AFI in code stands for Always False, an instruction commonly used in Programmable Logic Controller (PLC) programming, particularly within industrial automation systems like those developed by Rockwell Automation.
Understanding the AFI Instruction
The Always False (AFI) instruction is a fundamental program control instruction designed to manipulate the flow and state of logic within a PLC program. Its core function is to ensure that a specific condition or output remains false, regardless of other logical inputs or program states.
Core Functionality
- Setting EnableOut to False: The primary action of an AFI instruction is to set its "EnableOut" or equivalent logical state to false. This effectively breaks the logical continuity of the rung or sub-routine it controls, preventing any subsequent instructions on that rung or logic path from executing or becoming true.
- Preventing Execution: When an AFI instruction is encountered, it ensures that any conditions or outputs that follow it on the same rung or beneath a certain logical threshold will not be enabled or activated. This is crucial for controlling program flow and ensuring safe operations.
Applications and Use Cases
The AFI instruction serves several practical purposes in industrial automation programming:
- Temporary Disabling: During program development, testing, or troubleshooting, AFI can be used to temporarily disable portions of code without deleting them. This allows programmers to isolate sections of logic or prevent certain outputs from activating during testing.
- Example: A new section of code for a motor control might be placed after an AFI instruction until it's fully tested and ready for deployment.
- Permanent Deactivation: In some cases, a programmer might need to ensure that a specific output or logical condition never becomes true. AFI can be placed at the beginning of a rung to guarantee its deactivation.
- Example: To permanently disable an old alarm output without removing the rung entirely for historical reference.
- Placeholder Logic: When designing a program, AFI can act as a placeholder for logic that is yet to be written. It ensures that the program compiles and runs without errors, even with incomplete sections.
- Forced Off Condition: It can be used to explicitly force an output or internal bit to an 'off' or 'false' state, overriding other logic that might attempt to turn it on.
Operational Context
It's important to understand that the behavior of AFI, particularly how it affects subsequent conditions, is often contingent on the PLC's operational mode. For instance, the impact of AFI on conditions lower down in a hierarchical program structure is typically observed when the PLC is operating in its Normal Scan mode. This ensures predictable and controlled behavior during standard machine operation, preventing unintended actions during specific programming or debugging states.
