One thing I would like to add to Ron's excellent write-up, is some safety advice when using forcing, and to explain a small, but significant, difference in force handling between the PLC5/SLC PLCs and Logix5000 controllers.
If you put forces in an Allen-Bradley processor, you then enable them, (obviously when you do this you enable all the forces you have entered). You will get a confirmation prompt from the software, which is right and proper, because the forces may affect the plant, process, or machinery the PLC is controlling.
You should always check the status of forces before you start to do any forcing yourself.
a> There could already be forces installed, and enabled. When you add your forces, which you may do in advance of actually needing them "to be ready", they will take effect immediately, with no prompting!!.
b> There could already be forces installed, but disabled. When you add your forces, and then enable them, you will also be enabling the ones that were left in the processor by someone else. Of course, this may be a totally inappropriate thing to do.
c> On the PLC5/SLC platforms, if you remove your forces one-by-one, the processor will automatically disable forces. This does not occur with the Logix5000 controllers. It is possible to leave the processor in a state where forcing is enabled, but there are no forces installed. Of course, if you start putting in forces in that state, they will take effect immediately, with no prompting!!.
d> Whatever the scenario is, when you Save the project, you will also Save the current forcing conditions. That may be an inappropriate thing to do.
In summary, I would recommend....
1. Forces are only used when absolutely necessary (if at all).
2. Forces should not be left in the processor "unattended".
3. Forcing should not be used for "long term" solutions. You cannot document a force !!
4. Check the existence and status of forcing before you start. If there are forces installed, but disabled, remove them - they weren't doing anything anyway, and you probably don't want to enable them without fully investigating why they are there!!.
5. If there are enabled forces installed, tread carefully, there will be no prompting, and you must remember which ones to remove when you take yours out.
It's plain to see what could happen if you aren't aware of all of the above.....