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

Damn Ron, Giving away free training again? Keep that up and no one will need to go to your excellent classes.

P.S. By the way, Ron covers this by lunch the first day as well as a whole host of other Myths about PLCs. I for one highly recommend his classes to anyone who likes the above explanation and wants to really know how a AB PLC works and interprets your code.

Mark Snodgrass

Thanks for the explanation Ron, (on forces and grammer) and I find it amazing how many people do get confrontational when I reply to their e-mail and use the apostrophe properly. I'm not trying to correct them, I just don't want to join them in being wrong.

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.....

from Mark Snodgrass:

Giving away free training again? Keep that up and no one will need to go to your excellent classes.

Click to expand...

I figure it won't hurt to offer up some free samples once in awhile ... I don't want to jinx it – but lately I've been having to turn down about 50% of the work that comes my way – mainly the onsite training gigs ... I'm suddenly 66 years old now and I'm having to think about an exit plan for the business ... if the idiots in Washington can just keep the economy on the rails for one more year I'll probably call it quits ...

anyway – thank you for the kind compliments ...

Ron,
Retirement isn't as much fun as is advertised. I would suggest that you continue working if that is what you enjoy. You could just scale down the hours even if you have to turn down 75% or more of the work.

greetings jrwb4gbm ...

no, believe me, it's not the "fun of retirement" that I'm really interested in – but rather a level-headed business decision ...

thanks to Archie Jacobs and his hard work in publishing my "preview" YouTube videos, my little one-man operation is now at the very top of the Google listings for PLC training ... if I can find a buyer for the business now (while I'm still healthy) I should be able to get the best price for it ... if I wait until later (when I'm old and decrepit) then the price would certainly be less ... worse yet, if I die before I sell it off, then my precious little wife would only be able to get a fraction of the total value by selling off all of my lab equipment ...

I've got a lot to think about ... so far it's not keeping me awake at night – but I've still got to come up with a game plan which makes sense ...

thank you for your thoughts ...

really wonderfull presentation.Expecting more like this

Got it, thanks.

Click to expand...

Hi-Jack: Here is one suggested solution to your problem.

you are basically correct in the way that you described it ...

whenever you use the programming software to "force" the PLC's inputs - the LED indicators on the front of the input module do not change state too ...

in simple terms - when you force the input ON - you should not expect the LED indicator on the front of the module to turn on also ...

TIP: it's easiest to think of each LED indicator on the input module as being "hardwired" directly to the actual terminal/screw - so that whenever electrical current flows through the screw - then the LED will light up ... there is a little MORE TO IT than that - but thinking of it that way will help you preserve your sanity ...

now then ...

at the risk of making things more complicated - when you "force" the PLC's OUTPUTS - then the LED indicators on the front of the OUTPUT module will indeed change to reflect the status of the outputs ...

and please note: the description I'm giving is for most Allen-Bradley systems ... the same ideas might not be correct for other brands ...

welcome to the forum ... and thanks for bringing up this seven year old thread ... seems like just a year or so since I wrote it ...