RsLogix 500 - Using Forces and Understanding the Difference Between Input and Output Forces
Forces allow you to override the physical state of a PLC input or output.
I'm going to give you a few personal views on when and how to use forces at the bottom of this lesson but first let's understand how to know if forces exist in a PLC, how to force an input or an output, and how exactly forces are used in the PLC.
How to tell if forces are installed.
You can tell if forces are installed or not by looking at the online toolbar of RsLogix 500. To the right of the PLC Mode that shows you if you are in Run or Program Mode, you will see either "No Forces" meaning no forces exist in the program or "Forces Installed" meaning that there are forces in the program. Also note that in the Micrologix PLC, forces are always enabled by default.
You can tell if an input or output is forced on or off while browsing the ladder logic by looking below the instruction to the left of the bit specifier. It will either show ">OFF" if forced off or ">ON" if forced on.
How to Force a PLC Input or Output
There are two easy ways to force a PLC input or output. First, you can right click an instruction that addresses the input or output in the ladder logic program and near the bottom of the menu you will see "Force On" and "Force Off". In the Micrologix PLC, these forces will be immediately installed and active when you do this. To remove the force, right click the instruction addressing the input or output again and you will see "Remove Force".
The other way is the the Force Files in the left pane. There is an O0 - OUTPUT force file and an I1 - INPUT force file. They look very similar to the Data Files above them in the left pane but instead of showing the current input or output state, they will show a "." meaning there is no force installed, a "0" meaning a "Force Off" is installed, or a "1" meaning a "Force On" is installed. You can type "." to remove a force, "0" to force an input or output off, or a "1" to force an input or output on.
How are input and output forces applied?
Understanding exactly when these forces are applied and that input forces are applied in a different manor than output forces is very important when troubleshooting PLC programs. Below is the PLC scan you learned in the RsLogix 500 - PLC Scan Cycle lesson earlier with the timing of the forces inserted. Input forces are applied after the inputs are scanned and do affect the input data box in the PLC program. Output forces are applied after the PLC ladder logic has been executed so it does not affect the output data box in the program. Our "How Forces Work in Allen Bradley RsLogix 500 Micrologix and SLC PLC. Input Forces and Output Forces" video clearly explains this so go watch it.
Tim's opinion of forces
I try to be really objective in providing this content and include a lot of "rules" that are not based in facts of how the PLC operates, however forces are something I see misused and unsafely used by new PLC programmers. As I state at the end of this video about forcing, I probably haven't used a force in years outside of making lessons about forces. And I think the big thing I see newcomers do is force bits because they can't figure out why the PLC program isn't doing what it is supposed to. The root cause of this is usually not fully understanding how a PLC scans and executes programs. So really concentrate on grasping these core concepts.
The other very important comment I would have about the use of forces is try not to leave forces installed on a machine. If I walk up to a PLC and I see a force light on, it is a red flag that there could be an unsafe condition.
Practice understanding the difference between input and output forces. Also the video below will be very helpful in understanding how each affects the PLC program. Next go to the RsLogix 500 - Math and Math Errors lessons.