PLC Logic Scan

Ladder logic programs are modeled after relay logic. In relay logic each element in the ladder will switch as quickly as possible. But in a program elements can only be examines one at a time in a fixed sequence. The ladder logic will be interpreted justify-to-right, top-to-bottom.

The ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, and then the output branched below it. On the second rung it solves branches, before moving along the ladder logic rung.