Lessons · Industrial control · rung 2 sees what rung 1 wrote
Rung 2 sees what rung 1 wrote
Rungs are solved in order, and each one uses the values as they stand at that moment -- including anything an earlier rung has just written.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
It is the single fact that makes ladder read differently from every text language, and it is what lets a whole chain resolve in one scan.
How to think about it
Write the program in the order the machine works, top to bottom. Then a bit made on one rung is available to every rung below it, in the same pass.
Worked example
[A] (Mid) [Mid] (Out)A off, then on. Out: 0 1 -- Out came on in the SAME scan A did.
Your turn
A rung can use a bit written by an earlier rung in the same scan.
the earlier rung must be it
Run a rung and watch the scan
The trap
This only works downwards. A rung that reads a bit written below it is reading last scan's value.