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Any references on how to read 984 style ladder logic?
Just been handed some ProWorx ladder diagrams and trying to figure out what some of the strange blocks mean...
By David Campbell on 21 January, 2004 - 12:09 pm

A customer has asked us to implement an operator training simulator which includes what appears to be an aging Modicon PLC for motor control (print-out dates to Oct 1999, the PLC rack has had a processor upgrade around that time).

I am more familar with the IEC 61131-3 standard and I have already investigated the posibility of trying to convert the ProWorx database to a Concept database but there appears to be no easy migration path (please correct me if I am wrong).

Any pointers to some documentation would be greatly appreciated.

Modicon 984ll does not solve in a rung by rung format.

A program can have 32 segments
You can have up to thousands of networks per segment.
One network consists of 7 x 11 nodes.
A node contains on contact or coil.
Some instructions can take two or three nodes.

The gotcha here is that the solve order is not left to right and row by row. The solve order is column by column.

Another gotcha is that Modicon has a strange legacy of not requiring power flow to a block to have it actually function.

Good luck.

By Bradley Hite on 22 January, 2004 - 7:37 pm

Hello David,

The conversion might or might not be possible. If the Modicon system is a 984 based system then you are out of luck because Concept does not support the 984 controllers. If the system is a Quantum based system then you can just upload the ladder logic into Concept's 984LL editor.

Bradley G. Hite
Intertech Inc
Teaching Practical Skills for a Technological World

By Fernando Capelari on 22 January, 2004 - 7:37 pm

That's right, there is not a conversion tool from Proworx projects to Concept projects.

In fact, even if it would exist, there is not a way to convert 984 Ladder Logic to IEC Ladder Diagram.
So, the unique choice is convert the application "manually"...

But keep in mind that this is not true just for Modicon PLCs, I would say that it happens for all PLCs in the market.

The 984 ladder logic should be straight forward. If it is used for a motor control system probably very few function blocks are being used.

Feel free to contact me and I can give you some assistance to get you going.

By Pablo Carbajal on 10 January, 2013 - 7:59 pm

Hi, do you mind if I ask you a question?

I have a 984 printout which seems pretty straightforward, except that, besides the "regular" normally open or normally closed contacts such as ---] [--- or ---]/[---, I have this:


and also

--N] [---

What is the meaning of the "N"?

Thank you,

I thought this to be the case, and indeed I confirmed it with an old hand in Modicon. they are Negative transitional contacts. An old Modicon 584 book describes them this way: "Transitional Contacts pass power for one scan whenever the discrete inputs to which they are referenced change to the proper state, either OFF to ON or ON to OFF."

Go to and download from the Quantum or Compact or Momentum directories the manuals 'Modicon 984 Ladder Logic Block Library'. These detail the operation of all available commands in 984 ladder logic.