Retentive data on Modicon 984

When a Modicon 984 loses power why does it lose all of its current data values, returning back to the last saved values? Is there a setting that turns on retentive data or something? I couldn't find anything like that in Proworx32.

If not, is there a way to programatically save the values to nonvolatile memory every hour or so?

I don't know about anyone else, but I see this as a major problem with the Modicon platform, which is also a problem on the newer Unity PLCs (3 calls to Modicon with no resolution yet). By default PLC's should be able to lose power and return right back where they left off, with same setpoints, etc.!

Thanks

984 plcs do not lose analog data after power loss.
It have happen to us a lot of time and all setpoints etc stay in their last value.
Digital data ist lost. But if you use latched coils (L) they will stay in the last state after a power loss. Of course if the logic before the coil is not 1 it will go to zero.

Robert

I have remembered !

On some of the Momentum and Quantum PLCs it is possible to save the program on a flash memory.
After a power loss and without battery the flash memory is loaded to the plc memory. All program changes made after saving in flash will be overwritten by the flash.
Because of that I never use flashmemory. A long time ago one of my PLCs started with a program that was some month old. I was very lucky that day that nothing serios had happen.

Robert

I don't know about anyone else, but I see this as a major problem with the Modicon platform, which is also a problem on the newer Unity PLCs (3 calls to Modicon with no resolution yet). By default PLC's should be able to lose power and return right back where they left off, with same setpoints, etc.!

Click to expand...

On the Unity based systems go to the tag data properties and tick the 'save' box. The values are then retentative. This is done on a tag by tag basis. You also need to go to the cpu properties and untick the MWi reset on cold start box

SCADA_Dude said:

When a Modicon 984 loses power why does it lose all of its current data values, returning back to the last saved values?

Click to expand...

This does not happen with a 984. The 4xxxx registers retain their values thru power cycle, coils however are not retentive unless latched as MRobert explained.
It would only return to last saved values if you did a program download...it this what is happening? A program download does overwite register values.

With a 984 system a program download should be a rare event...only after a CPU replacement etc.

Do your modifications online!!

an after thought...there could be some PLC code that is writing default values to your registers on first program scan.

Thanks for the info, I will check to see if there is first scan code in the ladder. I didn't write this code, so I am unfamiliar with parts of it.

Also, with Unity I tried the MWi check box which you have to do offline. When I downloaded the program with this new setting, all my variable values were set to zero! Is there someway to load the old values after you do this?

Thanks

I thought u were programming a 984...but in last post u are referring to a Unity processor. They behave quite differently. So best to keep discussion on their behaviour completely seperate. What processor are u actually using a older Modicon 984 or a newer Quantum Unity or other. using generic "Modicon 984" is not enough.

Also, with Unity I tried the MWi check box which you have to do offline. When I downloaded the program with this new setting, all my variable values were set to zero! Is there someway to load the old values after you do this?

Click to expand...

Did you also tick the save box for each tag you wish to be retentative?
Also there is a function in Unity to save plc data to a file and another function to write the file back into the plc. Under the PLC menu you will find 'Transfer data from plc to file' and 'Transfer data from file to plc'

On Unity you can do %MWx to retentive, if you uncheck MWi reset on cold start.

On cold start bits are still reseted to zero, but words remains values before cold restart.


BTW, Does anybody know method, how I can save status of ebool bits, or do I have to use %MWi addresses also to make bits retentive?



Lare