How is this N7 updated and does the JSR?

[shaydzmi]

I have this program:
ZIP (.rss inside no documentation).
PDF (Italian documentation).
Allen Bradley SLC5/03 PLC.

I want to know how is the integer (N7:21) updated, in other words: How does it become equals to 10 and to 11 which leads to the correspondent SBR?

 

[JTCat]

If it is not coming from inside the program, it comes from outside the program.
Something else could be writing to that value, such as a HMI or messaging from another PLC or a PC application

 

[shaydzmi]

If it is not coming from inside the program, it comes from outside the program.
Something else could be writing to that value, such as a HMI or messaging from another PLC or a PC application

Are you confirming that it's not coming from inside the program?
Thank you.

 

[parky]

No JCat is saying it is possible that it is comming from somewhere else, could be that the value is updated from another source i.e. communication with other PLC or HMI, however, try doing a search for N7:21 in the whole project it could be written to elsewhere whithin the project, if it cannot be found by doing a cross reference then the only possibilities is that it is indirectly addressed (N7:[N7:23]) for example or it is from an external source.

 

[shaydzmi]

...

Hi sparky! I did exactly this:

try doing a search for N7:21 in the whole project it could be written to elsewhere whithin the project, if it cannot be found by doing a cross reference then

But as I learned from my other post it was an interrupt, and in another post it was a word, so maybe this time would be a phrase or a sentence or something else!

 

[JesperMP]

In file 8 rung 52, the italian comment is:
Quando forziamo la pagina 10 nella word N7:14(forzamento pagina) salto nella subroutine 6 dove andiano a caricare tutti i valori dei temporizzatori che si possono programmare.(file 8)
Translated to english:
When we force page 10 in word N7:14 (page forcing) we jump into subroutine 6 where they load all the values of the timers that can be programmed. (File 8)

It looks to me that the addres N7:14 was changed at some time to N7:21 without the comment being updated accordingly.
N7:21 is probably an HMI tag since it is not written to in the PLC code.

 

[drbitboy]

+1 for what @parky and @JesperMP have written.

When the value of N7:21 equals 10, Program File LAD 6 (U:6) is called, which writes several other values in Data File N7 (N7:100, N7:101, etc.) into various timer presets (T4:x.PRE). None of those N7 preset values appear to be written from within the program, at least not directly, so it seems most likely they are written from an HMI, which HMI also writes a 10 to N7:21 to trigger the call to Program File LAD 6.

There are a few ways this PLC program could write the value of a reference such as N7:21:

• directly e.g. [MOV 10 N7:21], [CLR N7:21 ADD N7:21 10 N7:21], etc.
  • In this case, a text search for the string N7:21 would locate such instructions
• indirectly e.g. [MOV 21 N7:0 ADD N7:0 -11 N7:[N7:0]]
  • In this case, a text search for the string N7:[ would locate such instructions.
• Block writes e.g. [FLL 10 N7:20 2] or [CPW/COP N7:0 N7:20 2]
  • In this case, a text search for one of the strings COP or CPW would locate such instructions.
• A message instruction, MSG, configured for e.g. Modbus, could read data from another device and write to an array of values that includes N7:21
  • In this case, a text search for the string MSG would locate such instructions
• N7:21 could be part of a FIFO array
  • In this case, a text search for one of the strings FFL or FFU or LFL or LFU or BSL or BSR would locate candidates for such instances.
• N7:21 could be part of a data block of a composite instruction such as a PID
  • In this case, a text search for the instruction, e.g. PID, would locate candidates for such instructions.
• There may be others

If all of the text searches above, including the ones we haven't thought of yet, are not present in the program, then as @parky and @JesperMP note, N7:21 is not being written to by the PLC program itself, so the source of any changes to N7:21 must be outside the PLC e.g. an HMI or an instance of RSLogix 500 in Online mode.

 

[Steve Bailey]

Based on the translation provided by Jesper, it is possible that there may be an HMI panel connected to the PLC configured to report which page it currently displays to the PLC in N7:21.
If the OP can simultaneously connect the programming software and the HMI, he can try changing to pages 10, 11, and 12 and watch to see if the value in N7:21 changes. also, he could look at the content of those HMI pages and see if they relate to the logic in the associated subroutines.
As Drbitboy and others have pointed out, there are a lot of ways to write a value to a PLC variable, some of them rather obscure. Finding which of the possibilities is active in any particular system can be a challenge.

 

[ndzied1]

Based on the translation provided by Jesper, it is possible that there may be an HMI panel connected to the PLC configured to report which page it currently displays to the PLC in N7:21.
If the OP can simultaneously connect the programming software and the HMI, he can try changing to pages 10, 11, and 12 and watch to see if the value in N7:21 changes. also, he could look at the content of those HMI pages and see if they relate to the logic in the associated subroutines.
As Drbitboy and others have pointed out, there are a lot of ways to write a value to a PLC variable, some of them rather obscure. Finding which of the possibilities is active in any particular system can be a challenge.

This was my thought on seeing Jesper's translation. It's a tough one to find if you didn't write the programs.

shaydzmi, please report back when you find more informaiton.