OSR instruction?

[plcsnet]

I read the explaination of the OSR instruction on this site, but I can't seem to get it to work as it is explained. On Scan 1 how can NC 1001 (Rung 2) AND NO 1001 (Rung 3) BOTH be True at the same time? I can see why NC 1001 on rung 2 remains True, but what makes NO 1001 on rung 3 go True? They both can't be True at the same time can they since one is a NO and the other is a NC? I can't see anything that has changed to cause the NO on rung 3 to go True.
If the NO 1001 on Rung 2 is False how can the NO 1001 on Rung 3 be True at the same time? This doesn't make sense!
The explaination also says that NO 0000 goes False after 100 scans. Why is this? Nothing in the instruction or on any rung I see will make it go False after 100 scans.

 

[Eric Nelson]

...but what makes NO 1001 on rung 3 go True?

Since rung 2 solved as true, bit 1001 is turned ON by the output instruction, therefore 'NO 1001' is now TRUE when rung 3 is analyzed.

The explaination also says that NO 0000 goes False after 100 scans. Why is this? Nothing in the instruction or on any rung I see will make it go False after 100 scans.

It might make more sense if you replace the sentence "After 100 scans, NO 0000 turns off (becomes false)." with "Let's say, after 100 scans, NO 0000 turns off (becomes false)."



-Eric