Make a file read only PLC5

[BeepBob]

Hello all,

I am seeing this behaviour where an integer file (N46:33), has an integer in binary (11110) which is 30 in decimal.

I did a search(cross reference) and the only place that reference N46:33 is a Compute instruction.


For some reason, after I download to the PLC, N46:33 , sometimes goes to 1111

I do not see any other instruction within the logic that can write to N46:33


Is there a way I can make N46:33 read-only?

 

[BeepBob]

I was thinkin slapping a MOV instruction in to force move '11110' into N46:33....

 

[Mickey]

Is N46:33 the destination (results) of the compute instruction?

 

[chelton]

Cross reference won't show usage if they are referenced by index/indirect or written via HMI,Scada etc.

 

[JohnCalderwood]

What is the CPT instruction - is it an expression?
If so, what is in that expression?

And the other N46 words around, are they similar?

Is there a power up /first scan routine that sets a value to the integer word(s)?

You cannot make it read only, but you can force a value into it.
The only other way is to check what other PLCs or SCADA/HMI could write values to it,.

 

[arpus4KM]

Look for inderect references for it. It will be somewhere in the program.

You can find it by searching for [ or ]

unfortunately in rslogix500 they didn't think to give a good way to find something that was indirectly addressed. just remember, you may be looking for something like

N46[XX] where XX is another address that sometimes becomes 33. and you could look for that value being put into it, or you could look for a file that matches the binary if they used a masked move to put a specific value in it.

If you want, just upload the file here and someone can look at it and tell you where it's being changed at.

 

[drbitboy]

If you want, just upload the file here and someone can look at it and tell you where it's being changed at.

This. PDF is best.

Are there any BSR instructions used in that program?

 

[TheWaterboy]

In addition to the indirect addressing, is a MSG from another PLC writing to this register? those won't show up either.