Modicon PLC addressing help...


Thread Starter



How is everyone here? I know that on the internet have a lot of high educated and experts kicking around, just like the expression of "crouching tiger hidden dragon" hoping someone can help me.

I am a PLC guy, but only have experence on the Rockwell software product. recently I have a contact with the modicon control, need a small program modification, it is a project for the city's recycling plant, which for the conveyor control, too bad, this is realy the 1st time for me to look into this type of PLCs. No idea how is the I/O or internal address such as B3. T4 C5, R6, N7, F8. Is there any one can help out?

Thanks in advance.
Modicon PLCs are arranged in coils, status bits and holding registers. Holding registers are word based and are in the 40,000 range and up. Coils are bit based and are from 1 to 9999. Status bits are bit based as well from 10000 to 19999. You will need to know which coils or holding registers you want to change.

We build a DF1 to Modbus converter that allows you to enter PLC type addressing and map it to Modicon type addressing. Check out our product line at
Nothing beats RTFM (Read The Fancy Manual). A quick look shows that the
only thing that the collection "B3. T4 C5, R6, N7, F8" has in common is
the fact that they do NOT conform to any known variable numbering
convention - legacy 984 ladder logic nor IEC 61131-3. The first has four
naming spaces 0XXXX, 1XXXX, 3XXXX and 4XXXX (XXXX - a decimal number),
the second has % in front of each variable type and number.
Conclusion - they are all cryptic variable NAMES given by the original

It would help if you mentioned what programming package was used -
Concept or ProWORX - to help you better.


Steve Myres, PE


B3, T4, C5, R6, N7, F8 are the default files in an AB controller (PLC, SLC, or Micro only; CLX uses tag based memory addressing). "B" is for bits, as in B3:0/2, "T" is for timers, as in T4:0.ACC, T4:0/DN, etc. He's not saying he has these tags in a Modicon, he's saying he doesn't know the Modicon memory layout for the items which correspond to those designations in an AB controller.

Michael R. Batchelor

So far as I know the B3, T4, C5, R6, N7, F8 conventions are only used by Allen Bradley. Modicon's, and everything else for that matter, are arranged *VERY* differently. So, your understanding of how ladder logic works will be fine, but learning to address specific I/O points of registers is going to be a completely new way of looking at things.

Michael R. Batchelor - Industrial Informatics, Inc.
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Just to add on to what has already been said Proworx is the better choice on software for any modicon PLC. If you are needing support you can also find more repair facilities and engineers that are accustomed to using proworx to assist you.

If you get into a pinch feel free to contact me at [email protected]