if you do not need the clock, then you can also store the values in an EEPROM (integrated in PLC). This does not require any additional HW.
To write to the EEPROM will create a DB. So your data must be in V-Memory.
1. Use SMB31 & SMW32. Max. 4 Bytes can be written, you can use it several times.
2. Use www.s7-200.net
and load the library free of charge.
3. If only a view data has to be stored, then you can write it to MB0..MB13 and select retentivity in the system block for this merker range. MB0 .. MB13 if retentiv, will be stred in the PLC internal EEPROM automatically.
4. If you do not change the values during operation, means you set the values only one time, then you can also use MW and select PLC; Create DB from RAM. This wsill generate a DB with the complete V-memory.
5. Use the battery to extend to 200 days.
Please be carefull with writting to EEPROM. max. write cycles 100.000. So dont write to EEPROM in each cycle.!!!!!
I hope this helps.