EFM

Retentive OTE

7 posts in this topic

Hi.

Is there a way to make/code an OTE to act like a retentive OTE?
So it will retain its state when the PLC is power cycled?
Similar to the the assignment symbol := in structured text.

It seems like what I'm looking for was discussed here. However they reference to code/attachments that doesn't seem to be there anymore.
http://forums.mrplc.com/index.php?/topic/7252-retentive-bits/

Edited by EFM

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OTL is retentive so you could write some code using OTL to keep the state of OTE during power cycle

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Keep in mind that you will need to pair each OTL with an OTU (latch and unlatch) in order to turn the output off.

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yeah agreed & make sure the program will still be safe if you are going to OTL

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I know about OTL and OTU, however then the logic have to be rewritten,
as the OTL needs to have an separate and specific unlatch logic for the OTU.
While a "retentive OTE" and := in ST, just need one and the same logic that will be true or false.

To ask a stupid question, is there a smart/simple way of using OTL and OTU to represent "retentive OTE" and := in ST?
I don't see it, but it seems like this was exactly what was discussed and had a solution in this link.
http://forums.mrplc.com/index.php?/topic/7252-retentive-bits/

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Ok I'm not really sure what the exact method Ron proposed way back when as it is no longer available.  In fact I'm kinda surprised that his posts all have "Guest" by them.  All of that aside, they do discuss something that I was going to suggest you try and it looks like one person has used it successfully in the PLC5.  Keep in mind that as we have mentioned earlier, all memory locations associated with OTE instructions will be reset upon power up or return from program mode.  The way you may get around this is to store all of your outputs in INT tags continuously in order to save their operating value.  Upon return from power up or run mode, you would use the first scan bit to copy from those storage registers into your output registers effectively restoring the output word values to your outputs.  Obviously this move instruction would need to be processed before any output instructions to work properly but otherwise it should work.  I've never tried this before and it sounds like a good way to get into trouble from a safety point of view.  You generally dont want things starting up from a power outage without some sort of operator intervention.  If you only have a few outputs that you need to do this with you could use a masked move to only enable the outputs you are interested in restoring on power up. 

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Thanks, I'll see how we end up with solving it.

I would just prefer to keep it in ST if we can, but it would be so much more simple, if there was an equivalent to := in ladder also.

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