Hello there …
Don’t trust too much in my opinion but as far as I know when you compress the memory what you actually are doing is to erase old objects which you reload already. Supposedly in the new SIEMENS PLC, this process is automatic, although not always is working … anyway, if you did it manually and it is not compressing, it means you are actually using all this load memory …then it means that if you try to load some object bigger that 7kb (100-88)%, you are not going to have enough memory.
I shouldn’t say that, but using VIPA what is the same than SIEMENS ( I have my doubts) I had a problem compressing, then I did a reset of the PLC, I reload the hardware configuration and it was working again … although not always is possible to do a total Reset.
A mmc128k use to be around 10-20$ more expensive than 64k …
Good luck.