Lessons · Industrial control · the two rails a rung sits between
The two rails
The vertical lines on each side of a ladder are the rails: power on the left, the return on the right. A rung is true when contacts form an unbroken path from left to right.
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
The diagram is a picture of the relay panels it replaced. Reading it as electricity flowing left to right is what makes series mean AND and parallel mean OR.
How to think about it
For any rung, trace a path from the left rail: through each closed contact, around any branch, to the coil. If a path exists, the coil is on.
Worked example
[Power] [Enable] (Pump)Power flows from the left rail, through Power, through Enable, into the Pump coil, to the right rail.
Your turn
Complete the path to the coil.
[A] [B] ()
Run a rung and watch the scan
The trap
Nothing real flows in a modern PLC; the rails are a notation. But designs that forget the picture put outputs on the left and conditions on the right, and nobody can read them.