Core Concept: From “Wiring Technician” to “System Architect”
First, please change a core concept. An excellent PLC programmer is not just an “electrician” who follows wiring diagrams, but a “system architect” who oversees the entire project. They must ensure that every brick (bit logic) is placed correctly, and design the entire structure (automation system) with a solid framework, nerves, and soul, making it stable, efficient, and intelligent.
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Part One: Methodology – The Three Realms of Mastery
First Realm: Building a Solid Foundation – “Understanding Your Tools”
Before you attempt to build a skyscraper, you must be thoroughly familiar with the bricks, tiles, and tools at your disposal. For PLC programming, this foundation consists of Ladder Logic and Structured Programming.
· Metaphor: Learning Language and Grammar
· Ladder Logic (LAD) is like pictographs, intuitive and visual, especially suitable for those with an electrical background. You can “see” the flow of electricity, just like viewing a simple circuit diagram. It is the “dialect” you use to communicate with machines, friendly and easy to start with.
· Statement List (STL) and Structured Text (SCL) are akin to formal written language or programming languages. They are more concise and powerful, capable of performing complex mathematical operations and logical judgments. To write elegant and efficient code, you must master this “official language”.
· Core Methods:
· Master Basic Instructions: Like memorizing vocabulary. Bit logic, timers, counters, comparison instructions, etc., must be mastered to a level of “reflexive response”. These are your “bricks”.
· Master Function Blocks (FC/FB): These are your “prefabricated components”. Do not pile all logic into the main program. Package repetitive functions (like motor control, valve actions) into function blocks. Imagine an architect who does not make bricks every time but uses pre-made doors, windows, and beams. This greatly enhances efficiency, reliability, and maintainability.
Second Realm: Entering the Hall – “Building an Ordered Kingdom”
Once your “vocabulary” is sufficient, you need to learn how to write beautifully structured “articles” or “symphonies”.
· Metaphor: The Conductor of an Orchestra
A complex automation system is like a symphony orchestra. You have strings (motors), woodwinds (cylinders), and percussion (sensors). As a programmer, you are the conductor.
· The Main Program (OB1) is your baton, determining the flow of the piece (scan cycle), coordinating the entry and exit of various sections (function blocks).
· Organization Blocks (OB) are the conductors of special rhythms. For example, an Interrupt OB is like a soloist’s cadenza in the orchestra; when an urgent signal (interrupt) arrives, it can immediately interrupt the regular performance, prioritize processing, and seamlessly resume afterward.
· Data Blocks (DB) are the music stands for all musicians. They hold the pitch and rhythm of each instrument (device parameters, status). Classifying and storing all data orderly in different DBs, rather than scattered, allows any musician (function block) to quickly and accurately find the information they need.
· Core Methods:
· Adopt Modular Design: Divide the system into “feeding modules”, “processing modules”, “detection modules”, etc. Each module is programmed and debugged independently. This is like constructing a building, first completing the foundation, then the main structure, and finally the decoration, in an orderly manner.
· Plan Data Flow: Clearly define which are “global variables” (orchestra common rules) and which are “local variables” (musician personal habits). Ensure that data transfer between modules is clear and controllable, avoiding “data noise”.
Third Realm: Integration – “Endowing the System with Core Intelligence”
The highest level of programming is not just about making machines move, but making them move intelligently.
· Metaphor: The Diagnostic Skills of a Technician
An excellent program should resemble an experienced technician.
· “Look” (Powerful Diagnostic Function): The program needs to have extensive status monitoring and diagnostic logic built-in. When a device malfunctions, it should not simply stop; it should, like a technician, “see” that something is wrong and clearly inform the operator on the touchscreen: “Motor 3 overload, possible cause: conveyor belt jammed”, rather than a cold “fault code: 103”.
· “Listen” (Detailed Status Management): The program should be able to sense subtle states of the device. For example, a motor should not only have “start/stop” states but also “ready”, “running”, “warning”, “fault”, and “maintenance reminder” states.
· “Touch” (Flexible Process Adaptation): By externalizing process parameters (like temperature, pressure, time) through Recipe Data Blocks (Recipe DB), when changing products, you only need to call different “methods” without modifying the program. This endows the device with high flexibility and intelligence.
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Part Two: Steps – The Architect’s Construction Process
1. Blueprint Drawing (Requirement Analysis and Planning)
· Communicate deeply with mechanical and electrical engineers to thoroughly understand the process. Sketch the entire system’s action flow, safety interlocks, and human-machine interaction on paper or in your mind. This is the most crucial step, determining whether your final construction is advanced.
2. Foundation and Framework (Hardware Configuration and Framework Building)
· In TIA Portal, configure your PLC, IO modules, drives, etc., like building blocks.
· Create the basic framework of the program: design the main loop OB, initialization OB, interrupt OB, and plan the tree structure of FC/FB/DB.
3. Wall Construction and Decoration (Modular Programming and Debugging)
· According to the blueprint from the first step, implement functions module by module. Start by writing the most basic and core functions (like controlling a single motor), and then build more complex logic based on that.
· “Unit Debugging”: After completing each module, test it on a simulator or actual device to ensure that this “wall” is solid.
4. System Integration and Optimization (Overall Testing and Fine-Tuning)
· Combine all modules together for a full process test. The focus at this stage is to resolve “interface” issues and timing coordination between modules.
· Optimize program performance: Check for redundant logic, whether the scan cycle is too long, and if memory usage is reasonable. This is like the final cleaning and landscaping of a building, enhancing overall quality.
5. Delivery and Legacy (Documentation and Comments)
· Detailed comments are your “user manual” and “architectural drawings” left for future maintainers (or yourself three months later). Every complex logic segment and every custom function block should be explained in writing. A program without comments is like an article without punctuation.
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Part Three: Principles and Analysis – The Combination of Dao and Shu
· The “Core” Principle of PLC: Cycle Scanning
· Metaphor: The Diligent Postman
You can think of the PLC’s CPU as an indefatigable postman. He is always executing a fixed delivery route (scan cycle):
1. Check the Mailbox (Read Input Image Area): First, he goes to the main mailbox (input module) to collect all incoming letters (input signals, such as button presses, sensor statuses) at once, placing them in his notebook (input image area). During this delivery cycle, even if new mail arrives at the external mailbox, he will temporarily ignore it until the next round.
2. Process the Letters (Execute User Program): Then, based on the information in his notebook and the “processing manual” (user program) you provided, he decides which replies to write and what actions to take.
3. Deliver the Replies (Write to Output Image Area): After processing, he writes all the replies (output signals, such as turning on lights, starting motors) and places them in the “to be sent” area (output image area).
4. Unified Dispatch (Output to Modules): Finally, he sends all letters in the “to be sent” area to the corresponding external mailbox (output module), truly driving external devices.
Understanding this “core principle” is crucial! It explains why program logic is “synchronous” and why instantaneous changes in input signals may be “ignored” within the current cycle.
· Programming “Aesthetics” Analysis: Clarity Over Cleverness
· A program that appears clever but is structurally chaotic is far inferior to one that seems plain but has clear logic and complete comments. Maintainability is the primary productivity. Machines may not care if the code is beautiful, but the next engineer who takes over the program will. Write your program like an excellent essay: clear main idea, distinct paragraphs, and precise wording.
In summary, improving Siemens PLC programming skills is a journey from “technique” to “art”. It begins with mastering tools, progresses to grasping architecture, and culminates in injecting core intelligence into the system. Practice more, analyze more, and enhance your programming methods!
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