Reading ladder logic

Ladder logic looks like a wiring diagram on purpose: it was designed so the electricians of 1970 could read the new computer the way they read relay panels. Fifty years later it's still the floor's common language.

The anatomy of a rung

Power "flows" from the left rail toward the coil on the right — but only through contacts that pass:

• -| |- Examine if closed (XIC): passes when its bit is ON. Think "is the button pressed?"
• -|/|- Examine if open (XIO): passes when its bit is OFF. Think "is the button NOT pressed?"
• -( )- The coil (OTE): the rung's output. If power reaches it, its bit goes ON this scan; if not, OFF. No memory, no delay — it simply follows.

Series and parallel are AND and OR

Two contacts in a row: both must pass — that's AND. Two contacts stacked in a parallel branch: either path works — that's OR. Every ladder program, however huge, is these two shapes nested.

The trap: -|/|- is not "the stop button"

The most common reading error on the floor: -|/|- doesn't mean "a normally closed button is wired here." It's a question about a bit in memory — "is this bit OFF?" — regardless of what hardware feeds it. A stop circuit usually combines an NC physical button with an XIO instruction, and keeping those two ideas separate is what makes fail-safe wiring make sense.

Why this matters

Contacts are questions, coils are answers, and the scan asks every question thousands of times a minute. Next lesson: the one rung that runs half the motors on Earth — where a coil answers its own question.