Conceptual question - Siemens S7-300

[_m1ke]

Hi there, I have trouble understanding the "mechanics" of STL programming language for Siemens S7-300 PLC. For instance I have a code which looks like this (taken from the Siemens manual for extended pulse timer): With regards to the first line, AND operator checks whether the state of I 2.0 is '1'. According to the "Bit Logic Instructions" manual, it also ANDs the test result with RLO. Now the question is - what RLO is it if there are no operations beforehand? What memory area does it use for that particular RLO? Moreover, what is the Boolean bit logic string in this case? I presume it is all the lines up to the '=' sign. The confusion is also caused by instructions (FR, L, T) which are not logical operations or RLO, although there is associated memory bit with them. I'd be grateful if you could clarify this matter? Many thanks!

[Moggie]

This might help (a little) - http://www.plcdev.com/book/export/html/632

[_m1ke]

Thank you, that explains a lot!