Why can't I force when Forces are Enabled [ControlLogix L73]

Hi there,
Can I only force outputs to stay Energized/On or De-energized/Off or does this work within a ladder logic program on temporary coils too ?
Are there circumstances under which, when Enable Forces is true, that Logix 5000 would not let me do forces?

In the ControlLogix platform (really any A-B platform), "Forcing" only modifies the actual Input and Output tags directly related to a physical I/O module.

You cannot "force" an "internal" tag that is not directly related to the physical I/O.

Forceable: Local:2:O.Data[3]

Not Forceable: MyLocalOutputData.3

Thanks again Ken, appreciated ... I figured that out a year ago but haven't been at "this" since ... guess its true about losing what we rarely use.

it turns out that "forces" aren't nearly as simple as most people believe them to be ... the link below might be useful to you ... there are several figures at the beginning that have helped a lot of folks allay some very common misconceptions ...

the title says a lot ...

Forces in Allen-Bradley – more than you really wanted to know

http://www.plctalk.net/qanda/showthread.php?p=537414&postcount=1

be safe ...

What I frequently do for troubleshooting is add a branch with a XIC ToggleOnToForce tag.

Then I can toggle that bit on to force the line true (whether an output or a memory bit)

I also put just before the output a XIO ToggleOnToForceOff tag, in case I want to force the line false.

And, sometimes, I remove those after commissioning the control.

Personally I hate forcing the physical IO, don't like it when I get online with a new PLC and find forces existing and enabled. I generally do as above and remove the forces.

As Ron's link shows, a forced output shows during the scan as OFF sometimes, and if checked elsewhere in the program to do something else if it is ON then the program will see it OFF and not do what it is supposed to do. [Say, run a replenish system or pH monitor when a main pump is running.]