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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.
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Forcing I/O

Forcing can be used for troubleshooting, and to some extent simulates real world jumpers. Leaving forces in the processor, or depending on forced I/O to make your equipment run is considered bad practice.


Look at the diagram below:



Under normal circumstances, the following events take place:

  1. The switch is shut

  2. A 1 appears in the input tag

  3. The XIC instruction goes true

  4. The OTE is enabled

  5. A 1 is written to the output tag

  6. The light will energize on the output module


Forcing the input:

If you place a jumper across the switch, you would have the same effect as the switch always being shut. A 1 would always be in the data table, the logic would be true, and the light would energize. The same effect applys to forcing. Forcing the input on would result in a 1 in the input data table for the switch, and all logic would be executed as if the switch was shut. The opposite applys to an OFF force. An Off foce would be similar to cutting a lead on the switch. A zero would result in the input data table.


Forcing the output:

If you place a jumper to the output, the output table would still be a zero if the logic is false. Information does not flow from the output device to the output data table. Therefore, any XIC instruction that is looking at the output bit would also be false. The same applies to forcing. If you force an output device, the output data table will still be controlled by the ladder logic.


Note: Even though forcing an output does not directly effect the data table, The field device itself could feed an input back into the processor causing other things to happen in logic. Know your system before using the force feature.


There are several ways to force I/O. Forcing can be applied from ladder logic, or from the Controller Tag database. Internal memory locations cannot be forced. You can only force real I/O, Aliases to real I/O, or producer/consumer tags.


In this example, we will force an input directly from ladder logic. Right click on the input address, and choose 'Force On'.





Notice the force light on the processor begins to flash (if your process has a force light) indicating that forces are installed, but not enabled. The force can be enabled from the online toolbar as shown below:





The force light on your processor will now be solid amber indicating that installed forces have been enabled. If we go to the data table, you will see that the input bit is on, and it is red indicating that a force has been enabled on the input. You will also see that the forc mask reflects which bits have been forced in the data word. Forcing can be preformed directly from the force mask as well.





In the force mask, the value of 1 indicates a bit has been forced on. The value of 0 indicates an off force, and a period indicates no force is installed on a particular bit.