Siemens TIA Portal – Asset Oriented Programming - Structuring Sequences In SCL
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In this lesson, we take a detailed look at the wash sequence manager and compare how it is implemented in ladder logic versus structured text. The focus is on understanding the differences in approach, structure, and day to day usability when managing a complex sequence in different TIA Portal languages.
The wash sequence is more involved than the other modes because it is made up of an overall governing sequence with several functional sections. These include drain down, fill, air scour, backwash, top wash, and rinse. The lesson explains why these sections are handled inline within a single sequence rather than being broken out into separate function blocks, and when it would make sense to do that instead.
You are walked through the full wash process from start to finish. This includes how wash mode is called, how filtration mode is safely shut down when required, and how fault shutdown conditions are handled. Particular attention is given to level control around the wash weir and why managing the level correctly is critical to protecting the filter media.
The lesson then moves into the structured text implementation using a case statement to manage the sequence steps. You will see how variables are initialized, why this is required in structured text, and how requests must be explicitly reset when they are no longer needed. Timers, internal shutdown requests, and fault handling are all explained in detail.
Each phase of the wash sequence is covered step by step, including air scour timing, backwash behavior, top wash flow, and the rinse process with turbidity checking. The logic behind repeating the rinse when water quality is not acceptable and triggering a failure when limits are exceeded is also explained.
The lesson also covers how the wash sequence interacts with the filtration sequence using interlocks, allowing the wash cycle to interrupt filtration and then hand control back cleanly once complete. This loose coupling approach is highlighted as a robust way to manage interruptible sequences.
Finally, shutdown management is reviewed, showing how all valves are closed, alarms are raised, and the sequence is safely returned to idle. The lesson concludes with a discussion on the practical differences between ladder and structured text, especially around scanning behavior, monitoring, and debugging.
This lesson provides a clear and realistic walkthrough of a real world wash sequence implemented in structured text, helping you understand both the logic and the practical considerations when choosing between ladder and SCL in Siemens TIA Portal.
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