Understand PLC programmable controller
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How do you understand a PLC as a technician?
Four components and one cycle. The components: the CPU that executes logic, the memory that holds program and data, the I/O modules that connect sensors and actuators, and the power supply that feeds them. The cycle: read inputs, execute the program, write outputs, repeat, continuously and deterministically. Everything else in industrial control builds on that model, and the three languages a technician meets first (ladder above all) express logic directly on top of it.
The four components
| Component | Job | Details that matter |
|---|---|---|
| CPU | Executes the program | Runs the scan cycle; holds the real-time behaviour of the whole system |
| Memory | Stores program and data | Program memory for logic, data memory for tags and values; retention scheme varies by family |
| I/O modules | Connect the field | Digital inputs from sensors and switches, digital outputs to motors and valves; analog modules for continuous signals |
| Power supply | Feeds the system | Converts mains to logic-level DC; sized with margin or faults follow |
Scroll the table horizontally to view all columns.
The scan cycle
The PLC works in a repeating loop: read all inputs into the input image, execute the program from top to bottom against that image, write the output image to the field, then housekeep (communication, diagnostics). The cycle repeats continuously, and its bounded, predictable duration is what makes a PLC suitable for machines. One consequence is worth internalising early: a coil updated late in the program affects the field only after the output scan, which is why program order matters even when the logic looks correct.
Example: the mental model in one sentence
When a limit switch changes state mid-scan, the program does not notice until the next input scan, and the motor it controls moves only after the next output scan. Technicians who carry that one sentence diagnose half of the "it behaved randomly" faults they will ever meet.
The languages you will meet
| Language | Abbreviation | Typical use |
|---|---|---|
| Ladder logic | LD | The most widespread style, and the natural transition for electricians: it mirrors relay diagrams |
| Function block diagram | FBD | Process control and signal-flow thinking |
| Structured text | ST | Math-heavy and data-heavy logic |
| Sequential function chart | SFC | Step-by-step machines and batches |
Scroll the table horizontally to view all columns.
Start with ladder: it is the most common, the most visual, and the one the maintenance floor reads. Scan-time figures vary by controller and program size (from fractions of a millisecond to tens of milliseconds); speed-critical applications need the fast end, which the controller's documentation states per model.
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