In PLC programming, an "addon" typically refers to a custom, reusable block of logic designed to simplify and standardize code. A prominent example, especially in Allen-Bradley PLCs, is the Add-On Instruction (AOI).
Understanding Add-On Instructions (AOIs)
Based on the provided reference, an AOI is a specific type of "addon" functionality:
AOI stands for Add-On Instruction which is a type of function implementation in Allen Bradley PLC programming. An AOI allows the user to create a custom function with inputs and outputs and to create instances of the functions to be used in the application created in RSLogix / Studio 5000.
This definition highlights the key aspects:
- Custom Function: You create your own specific logic (e.g., controlling a motor, calculating a value, managing an alarm).
- Inputs and Outputs: Like a standard instruction (e.g., Timer, Counter), an AOI has defined connection points for data to flow in and out.
- Reusable Block: Once created, the AOI can be used multiple times throughout the PLC program.
- Instancing: Each time you use an AOI, you create an "instance" of it, which has its own unique set of parameters and internal memory, allowing it to operate independently.
- Platform Specific: The term AOI is specifically associated with Rockwell Automation's Allen-Bradley PLCs using software like RSLogix 5000 or Studio 5000.
Why Use Add-On Instructions (AOIs)?
AOIs offer significant advantages in PLC programming:
- Modularity: Break down complex programs into smaller, manageable blocks.
- Reusability: Write code once and use it for identical or similar tasks throughout the project or even in future projects.
- Standardization: Ensure consistent control logic for similar pieces of equipment or processes.
- Simplified Main Routines: The main program becomes easier to read as it consists of high-level calls to these custom functions instead of extensive ladder logic or structured text.
- Easier Maintenance and Troubleshooting: Changes or bug fixes only need to be made in one place (the AOI definition) rather than in every instance it's used.
Practical Application
Imagine you have 10 motors to control, each requiring similar start/stop logic, fault monitoring, and status indication. Instead of writing the same complex logic 10 times, you can:
- Create one Motor Control Add-On Instruction (AOI).
- Define its inputs (e.g., Start command, Stop command, Motor feedback) and outputs (e.g., Motor Running status, Fault status).
- Place 10 instances of this AOI in your program, one for each motor, connecting the relevant tags to its inputs and outputs.
This approach drastically reduces programming time and potential errors.
In summary, while "addon" can be a general term, within the context of many modern PLC platforms, particularly Allen-Bradley, it strongly correlates with the concept of Add-On Instructions (AOIs) – custom, reusable function blocks that streamline programming and improve code quality.
