Siemens S7-1200 System Manual page 397

Example Data log program
Example data log names, header text, and the MyData structure are created in a data block.
The three MyData variables temporarily store new sample values. The process sample
values at these DB locations are transferred to a data log file by executing the DataLogWrite
instruction.
Network 1 REQ rising edge starts the data log creation process.
Network 2 Capture the DONE output from DataLogCreate because it is only valid for one
scan.
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
Extended instructions
8.7 Recipes and Data logs
397