Creating retentive tag in Siemens

I have a small project running on a S7-1500. In the program is a bit %MD66 which is a value input from an HMI. I want to make this value retentive on the HMI when power is cycled. How do I go about doing this? Thanks

I can't find it in the manual but the M area does not appear to be retentive for the S7-1500 (it is selectable in the s7-300).


Use a tag in a DB instead and make that tag retentive.

Just like in the 300's, M memory can be made retentive in a configurable range, MB0 to whatever you configure. By default, no tags are retentive. To make some memory retentive, use the Retain icon on the tag table toolbar. It's the one with a wrench on the right side.

That said, I agree with LD. DBs are simpler and more flexible. The best practice I generally hear is to use M memory as little as possible, besides things like the clock bits. I find that M memory is great for tags created for testing. It's easy to then go back and look, if there are any PLC tags, you still have some testing code left.

M area is a little dangerous because of the bug in automatic tag creation in v14. That does not check for defined bytes and will create overlapping words. Or maybe it’s a setting I’ve missed. Saw it once and made my mind up because it’s TIA.

Thanks for the input. I used a global DB to create a separate tag and made it retentive. Replaced the M tag with that.