Working principle of programmable controller and input and output processing principles
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What is the working principle of a PLC, and how are inputs and outputs processed?
The working principle is the scan cycle: read all inputs into an image table, execute the program against that image, write the output image to the field, then run housekeeping (communications, diagnostics). Inputs are processed by type: digital inputs are read as on/off states, analog inputs are converted from 4-20 mA or voltage signals to engineering values, thermocouples add cold-junction compensation, and encoders feed high-speed counters. Outputs come in relay (mechanical, slower), transistor (fast, DC loads), triac (AC loads) and analog forms, each matched to the load it drives.
An earlier version of this page opened with teaching-experience claims that could not be verified. They have been removed.
The scan cycle
| Phase | Action | Notes |
|---|---|---|
| Input scan | Read all physical inputs into the process-image table | The program sees this image, not the live inputs |
| Program execution | Run the user logic, rung by rung, against the image | Duration depends on program size and instructions used |
| Output scan | Write the output image to the physical outputs | The field updates once, after the logic pass |
| Housekeeping | Communications, self-diagnostics, clock | Can extend the cycle on busy networks |
Scroll the table horizontally to view all columns.
Two consequences follow from this model and explain most beginner confusion. First, a program that evaluates the same coil twice gives the last write precedence, because the output image is only written once per scan. Second, an input that changes mid-scan is invisible until the next input scan, which is why fast events belong on high-speed counter or interrupt inputs rather than on ordinary rungs.
Input processing principles
| Input type | Processing | Typical application |
|---|---|---|
| Digital | On/off state into the input image | Proximity sensors, push-buttons, limit switches |
| Analog (voltage / current) | Converter produces a numeric value the program scales | Temperature, pressure, level via 0-10 V or 4-20 mA |
| Thermocouple / RTD | Dedicated modules with reference-junction compensation | Ovens, furnaces, cold rooms |
| Encoder / pulse | High-speed counter inputs, bypassing the ordinary scan for fast pulses | Position and speed measurement on motion |
Scroll the table horizontally to view all columns.
Output processing principles
| Output type | Switching | Notes |
|---|---|---|
| Relay | Mechanical contact | Slower switching, accepts AC and DC loads; contacts wear with cycles |
| Transistor | Solid-state, fast | DC loads; inductive loads need suppression protection |
| Triac | Solid-state AC switching | AC loads only |
| Analog | Proportional current or voltage output | Scaled values drive VFDs, proportional valves, recorders |
Scroll the table horizontally to view all columns.
Program structure influences scan time more than CPU speed alone: unused routines, deep nesting and redundant logic all lengthen the cycle. Cleaning up the program regularly is a performance measure in its own right.
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