Basic Instructions and Scan Cycle
In this lesson we are going to learn the most basic PLC scan. We will get into greater depths of this in later lessons.
The PLC executes three basic steps.
1. Reads the physical inputs and updates the input data table putting a 0 in the cells whose inputs are OFF and a 1 in the cells whose inputs are ON.
2. The processor evaluates the ladder code from left to right and top to bottom, reading data table values as needed, and updating data table values as the code requires. Note that the PLC does not read inputs or turn on outputs. It simply reads and writes to data boxes.
3. Updates the physical outputs based off of the output data table turning OFF outputs whose output data table values are a 0 and turning ON outputs whose output data table values are a 1.
Overhead - There is a small overhead step where the PLC does various housekeeping steps such as communications with your PC then it returns to step 1.
Whenever you are trying to understand what the PLC is doing, always come back to this basic concept. A PLC always follows a very methodical step-by-step process that will always have a predictable outcome.
