Understanding the ControlLogix operating cycle

Historically, PLCs such as the SLC-500 and PLC-5, and even ControlLogix controllers such as the Series 5 (L5), execute sequentially in a predictable, single-threaded manner, as illustrated here:

Newer models of the ControlLogix family, such as Series 6 (L6), Series 7 (L7), and Series 8 (L8), perform much differently than the previous synchronous PLC scan example. Newer ControlLogix versions take advantage of the performance increases afforded by the multithreaded asynchronous operation.

Asynchronous operation simply means that the controller will execute multiple tasks at the same time and will not wait for a previous task to complete before continuing to the next task (as the following diagram illustrates):

The trade-off for the performance gained, however, is some level of unpredictability, where the inputs and outputs are now continuously scanned while the logic executes. This has the potential to create a situation where the state of the process can change midway through the logic execution. We will look into this in Chapter 8, Writing Ladder Logic, in more detail, and also discuss a commonly used buffering technique to protect code from unpredictable state changes.