Quick answer: an Arduino can run a PLC-style loop – read inputs, evaluate logic, write outputs, repeat – and that is a genuinely useful way to learn PLC concepts. It stops being “a PLC” the moment protection, isolation and certification enter the picture, and none of those come from the microcontroller alone.
The scan cycle in plain terms
PLC programming ideas map onto a simple loop: gather input states, evaluate your logic against them, then drive outputs, and do it repeatedly. Keeping those three phases distinct in your code makes the behaviour predictable and easier to reason about than interleaving reads and writes.
Simulated controller versus industrial PLC
Proteus VSM co-simulates supported microcontroller firmware with mixed-mode SPICE circuitry, and the vendor documents interactive indicators and debugging aids. The vendor is also explicit that simulation is distinct from physical prototype testing – which is exactly the boundary a hobby “PLC” sits on.
| Aspect | Microcontroller + simulation | Industrial PLC |
|---|---|---|
| Execution | Software loop, timing depends on code | Deterministic, real-time scan |
| I/O protection | Not inherent to a bare MCU | Opto/relay isolation, surge handling |
| Safety functions | Not implemented | Emergency-stop and safety relays |
| Validation | Simulation only | Physical prototype plus standards testing |
An ordered I/O mapping workflow
- List every input and output by function before naming any pin.
- Assign each signal to a physical point and record the mapping in one place.
- Note the electrical character of each point – digital or analog – and keep analog source impedance within the datasheet’s guidance of about 10 kilohms or less.
- Remember the ADC is 10-bit with a single-ended result referenced to the selected VREF, so define that reference explicitly.
- Simulate the logic and confirm each output responds to its inputs, treating the result as a logic check rather than a physical validation.
Two datasheet cautions are worth repeating here because they are easy to violate in automation code: the ADC’s absolute maximum ratings are stress limits rather than functional operating conditions, and the ATmega ADC reference and source-impedance guidance are not optional niceties.
Reader questions
Is this actually a programmable logic controller?
It is a programmable logic concept running on a microcontroller. Real PLCs add isolation, deterministic scan timing and safety functions this does not have.
Can I use it to control mains machinery?
No. This is a low-voltage, simulation-oriented reference; machine safety, emergency stops and mains automation are out of scope.
Does a clean simulation prove the design works?
No. The simulation vendor states it is distinct from physical prototype testing. Simulation validates logic, not wiring or protection.
Is a relay safe as a protective device?
Not by itself. Never rely on a microcontroller, or a controller-driven relay, as the sole protective shutdown.
Documentation
- Microchip ATmega328P datasheet, 7810D automotive edition
- Labcenter Proteus Virtual System Modelling documentation
See also the Arduino 4-channel relay using ATmega328P with Proteus simulation guide for a simulation-oriented companion read.
Documentation note: the cited sources cover MCU ADC characteristics and Proteus simulation capabilities. Ladder toolchain compatibility, relay schematic details and EMI immunity are outside their scope and are not covered here.
