Modicon 984 Ladder Logic

Saulo35

Member
Join Date
Jan 2012
Location
Port Coquitlam, BC
Posts
112
Hi there and happy 2013!

I was given a Modicon 984 printout. The file name of the .PDF file contains the string "Modicon 984 Proworx". I am attaching page 1 of said .PDF listing.

My question is: what is the meaning of the "N" letters preceding some of the normally open or normally closed contacts?

I am aware of positive and negative transitions, but I thing they would print as ]P[ and ]N[; plus, I don't think it makes too much sense to program more than one transition in series (the chance of capturing two transitions in the same scan must be close to nil).

I have been reading through Modicon manuals found online but could find no reference to the "N" letters.

Thank you,

Saulo35
 
I suspect that the "N" indicates that the I/O is forced (either on or off). Does your printout include the disabled table? If so, you may confirm (right or wrong)my suspicion. Is this a running program??? I've seen co-workers force contacts in an attempt to track down the cause of loss of permissives. I strongly recommend building traps rather than forces, ESPECIALLY when the words Gas and Pressure are involved.

Cheers,

Dan.
 
Thank you, Dan. A good suggestion, but I hope that's not the case based on the number of "N" letters I can see. Half of their inputs would be forced! OMG! However, I must concede that the "N" are attached to input (100XX) addresses only. Your may be right.

No, the listing does not include anything else other than ladder.
Yes, this program has been running for many years.
I agree 100% with your suggestion regarding traps. I have no access at this time to this PLC and/or site.
 
On Modsoft there were often Ns and Ps for positive and trailing edges or one shots. However I don't see the point of an N in front of a normally closed contact.
Given the suggestions you have got so far I would add a dummy rung to the code using unused bits just to test out what the N does.
 
Last edited:
Thanks to everybody.

I got a Technical Support Specialist at Schneider confirming that the "N" in front of a contact means it's forced ON, while an "F" means it's forced OFF.
 

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