Resources
Writing for people who program machines
The fundamentals first: scan cycles, timers, analog scaling, addressing. Those are the questions that get asked every week, forever. Then the ground where the answers online get thinner, which is where most of a working week actually goes: functional safety, operator screens, drives, fieldbus, instrumentation, panel building and the regulated industries. And what AI is and is not doing in this trade, written by people who have to make it work rather than sell it.
101 articles across 15 topics
SIL or PL: which one does your machine need?
Two standards, two scales, and one machine. Which one applies, how they map to each other, and why the answer is usually ISO 13849.
Answers: “What is the difference between SIL and Performance Level?”
Fundamentals9
What actually happens in one PLC scan
Read inputs, solve logic, write outputs, repeat. Plus the three details in that sentence that cause most of the bugs beginners write.
7 min read
The output image: why your coil did not work
A coil sets a bit in memory, not a terminal. Understanding when that memory reaches the real world explains a whole class of bugs.
6 min read
Normally open and normally closed, in the field and in the code
The most common wiring mistake in automation comes from a contact meaning one thing on a drawing and another in a rung.
6 min read
Seal-in or latch: two ways to hold a motor on
Both keep an output energised after the button is released. They fail differently, and that difference is the whole reason to choose.
7 min read
BOOL, INT, DINT, REAL: choosing the right type
Picking a type is a decision about range, memory and precision, and getting it wrong shows up as an overflow nobody sees.
6 min read
PLC or microcontroller: what actually decides it
An Arduino can switch a relay for a fraction of the price, and the reasons to buy a PLC anyway are mostly not technical.
6 min read
Retentive memory: what survives a power cycle
Which values come back after a restart is a design decision, and treating it as an accident causes machines to start on their own.
6 min read
The first scan bit and what belongs in it
One scan of initialisation, and a reliable way to make a machine start in a known state.
5 min read
What a PLC is, for people arriving from software
An industrial computer with an unusual execution model, and the constraints that explain every design decision in it.
6 min read
Instructions12
Sequencer instructions, and the state machine that replaced them
SQO, SQI and SQL, what they were for, why they are rare in new code, and what to write instead.
6 min read
FIFO and LIFO: tracking parts that are already moving
Load, unload, and the shift register they compete with. Where a queue is the right model, and where it silently loses a part.
6 min read
Bit shift registers, and the conveyor they were made for
BSL, BSR, the shift bit that must be a one shot, and why the pitch of the conveyor decides the length of the register.
6 min read
Indirect addressing: one rung instead of forty
Indexed access, the loop that replaces copied logic, and the out of range fault that takes the controller down.
7 min read
User defined types: naming the thing instead of the address
What a UDT buys, how to decide what goes in one, and the versioning problem on a live system.
7 min read
Add-on instructions: reuse that survives being copied
What an AOI is, where it beats a subroutine, the versioning trap, and the reason to keep them small.
7 min read
TON, TOF and RTO: which timer, and why it matters
Three timers that look interchangeable and are not. The differences show up in exactly one place: what happens when the rung goes false.
5 min read
Counters: CTU, CTD, and the reset everyone forgets
Counters look simpler than timers and cause more field problems, almost all of them traceable to the same two mistakes.
6 min read
One shots: doing something exactly once
The instruction that turns a held button into a single event, and the three places it is genuinely required.
5 min read
MOV, ADD and the rest: maths in ladder logic
Arithmetic in a rung is conditional, which changes how you have to think about it.
6 min read
Comparison instructions and the analog value that is never equal
EQU, GRT, LES and the reason comparing analog values for equality does not work.
5 min read
Subroutines: structuring a program somebody else can read
Splitting a program into routines is the cheapest readability improvement available, and there are two traps.
6 min read
Analog & I/O4
Analog scaling: turning 4-20 mA into something you can read
The arithmetic is two lines. The part that catches people is what the raw counts actually are on your controller, and what happens below 4 mA.
6 min read
Sinking and sourcing, settled
Two words that describe which way current flows, and a reliable way to stop guessing which one you need.
6 min read
RTD or thermocouple: choosing a temperature sensor
Two technologies with different failure modes, and a straightforward rule for which to use.
6 min read
Wiring I/O without creating problems for later
A short list of practices that make the difference between a panel that is easy to fault-find and one that is not.
6 min read
Platforms5
CODESYS: the PLC platform you are probably already using
Hundreds of hardware brands run the same runtime underneath, which matters more than most engineers realise.
7 min read
TwinCAT: a PLC that is really a PC
Beckhoff's approach puts the controller inside Windows, which changes what is easy and what is worrying.
7 min read
Open source PLC software: what actually exists
OpenPLC, Beremiz and matiec are real and usable. Knowing what each one is for saves a lot of time.
7 min read
Siemens and Allen-Bradley do not just differ in syntax
Moving between the two platforms is less about learning new mnemonics and more about two genuinely different mental models of what a program is.
8 min read
Virtual PLCs: control logic without the controller
Running the PLC as software on standard hardware is now a real product from major vendors, and the tradeoffs are clearer than the marketing suggests.
7 min read
Migration3
PLC-5 to ControlLogix: what the converter will not do for you
Rockwell's migration tool handles the mechanical part and leaves markers everywhere else. Here is what those markers actually mean.
9 min read
SLC 500 to CompactLogix: the smaller migration nobody plans for
Less famous than the PLC-5 route and more common, with its own set of things that do not carry across.
7 min read
Moving a program between vendors, honestly
There is no automated path from Siemens to Rockwell, and understanding why explains what to plan for instead.
8 min read
Languages3
The five IEC 61131-3 languages, and when each is right
One standard, five notations. Most projects use two, and choosing badly makes a program nobody wants to maintain.
7 min read
Structured Text for people who think in ladder
The translation is mostly mechanical, and four constructs cover the majority of real code.
7 min read
Sequential Function Chart: when your machine has steps
SFC makes 'which step are we on' a first-class question instead of something you infer from a pile of bits.
6 min read
Networking10
Unified namespace: the idea, and what it does not solve
One broker, one hierarchy, every system publishing and subscribing. Why it beats point to point integration, and the three things it does not fix.
8 min read
MQTT Sparkplug or OPC UA: pick by where the data is going
One is a session between two machines, the other is a broadcast to a plant. The comparison people make, and the one they should.
8 min read
Diagnosing a PROFINET network that mostly works
Device names, update times, the difference between a lost frame and a lost device, and the tooling that finds intermittent faults.
7 min read
IO-Link: what it is, and the two reasons to use it
A point to point digital link to a sensor, parameter storage on the master, and diagnostics from devices that used to have none.
7 min read
Choosing a switch for an industrial network
Managed against unmanaged, the features that matter for PROFINET and EtherNet/IP, and why the cheap one in the office cupboard causes trouble.
7 min read
Getting PLC data into a database without wrecking the controller
Four architectures, the one that puts SQL in ladder and why it goes wrong, and the buffering that survives a network outage.
7 min read
Edge computing in automation, minus the marketing
What actually belongs at the edge, what belongs in the controller, what belongs centrally, and the maintenance cost nobody budgets.
7 min read
Modbus, still everywhere after forty years
The simplest industrial protocol, why it survives, and the three things that catch people out.
7 min read
EtherNet/IP and PROFINET: same cable, different worlds
Both run on Ethernet and neither talks to the other, which is a fact of purchasing more than of engineering.
7 min read
OPC UA: what it is for, and what it is not for
The protocol that connects the plant floor to everything above it, and the reason it is not a fieldbus.
7 min read
Safety7
Categories B, 1, 2, 3 and 4, in plain terms
Five architectures, what a single fault does to each, and the practical wiring that goes with them.
7 min read
Safety relay or safety PLC: how to decide
One is a wiring decision, the other is a programming one. The count of safety functions, not the size of the machine, is what settles it.
7 min read
Stop categories 0, 1 and 2, and why an E-stop is not a guard
Three stop categories, what each removes, and the reason a normally closed E-stop reads true when everything is healthy.
7 min read
Programming a safety PLC without inventing anything
Certified blocks, restricted logic, and the discipline that separates a safety program from an ordinary one.
7 min read
Muting a light curtain without defeating it
Why muting exists, the sensor arrangements that make it safe, and the timing rules that stop a person walking through behind a pallet.
7 min read
Two hand control: types, timing and why it is often the wrong answer
Type IIIA, IIIB and IIIC, the half second rule, and the reason two hand control protects exactly one person.
6 min read
A machinery risk assessment that is not a formality
The sequence ISO 12100 asks for, why the hierarchy of controls is in that order, and the residual risk nobody wants to write down.
8 min read
HMI & SCADA7
ISA-101: the grey screen that finds faults faster
Why a high performance HMI is deliberately dull, what colour is reserved for, and the evidence behind it.
8 min read
Alarm rationalisation: how to cut a list nobody reads
The questions that decide whether something is an alarm, the priorities that survive contact with an operator, and the paperwork that makes it stick.
8 min read
Alarm floods: why one fault produces two hundred alarms
Where floods come from, the four mechanisms that suppress them, and why shelving needs an expiry.
7 min read
How many HMI screens, and what goes on each
Four levels, the navigation that connects them, and the test for whether a screen earns its place.
6 min read
HMI colour: what each one should mean, and how many you need
A palette an operator can learn, why red for running is a trap, and the accessibility problem nobody checks.
6 min read
Choosing an HMI panel size before you draw anything
Why a panel does not reflow, the resolutions behind the inch measurements, and the viewing distance nobody checks.
6 min read
SCADA or HMI: what the words actually distinguish
Scope, architecture and history. Why the distinction blurred, and the questions that still separate them.
6 min read
Drives & Motion6
The twelve VFD parameters that actually matter
A drive has hundreds of parameters and a dozen decide whether the motor runs properly. Which ones, in the order to set them.
8 min read
Six ways a drive commissioning goes wrong
Cable length, shielding, ramp times, the missing brake resistor, motor rotation, and the autotune nobody ran.
7 min read
VFD or soft starter: what each one is actually for
One controls speed, the other only controls starting. The cost, panel space and process questions that decide it.
6 min read
Encoders: incremental, absolute, and the wiring that fails
Quadrature, single turn and multi turn absolute, the difference between resolution and accuracy, and why the marker pulse exists.
7 min read
Tuning a servo without a control theory degree
The three loops, what inertia ratio does to all of them, and a tuning order that converges.
8 min read
Electronic gearing and camming, and when you need either
Following a master axis in a ratio, following it through an arbitrary profile, and the registration problem both exist to solve.
7 min read
Instrumentation5
HART: digital data on the same wires as the 4-20 mA
How a digital signal rides on an analogue loop, what it gives you that the current does not, and why most installations never use it.
7 min read
Choosing a level measurement that will still work in a year
Radar, ultrasonic, hydrostatic, capacitance and load cells, and the process property that rules each of them out.
8 min read
Flow meters: which technology, and what it will not measure
Electromagnetic, Coriolis, vortex, ultrasonic, turbine and differential pressure, with the conductivity, viscosity and straight run each demands.
8 min read
Loop powered transmitters, and the voltage budget nobody calculates
Two wires carrying power and signal, the compliance voltage arithmetic, and why adding a display stops the loop working.
6 min read
Intrinsic safety, zones, and what Ex ia actually promises
Hazardous area classification, the difference between intrinsic safety and flameproof, and the entity parameters that must match.
8 min read
Panel & Electrical5
Laying out a control panel that can be worked on
Segregation, heat, access and the wiring that has to be traced at two in the morning by somebody who did not build it.
8 min read
Sizing an enclosure for the heat inside it
Adding up the load, what an enclosure dissipates on its own, and the decision between a filter fan, a heat exchanger and a cooler.
7 min read
Electrical noise: where it comes from and how to stop it
Capacitive and inductive coupling, the separation that works, screen termination, and the analogue input that reads correctly until the drive starts.
7 min read
Earthing and bonding, and the difference between them
Protective earth, functional earth, the star point, and why an earth measured at 0.4 ohms can still cause a fault.
7 min read
IP ratings and NEMA types, and which one to specify
What the two digits mean, the NEMA equivalents that are not equivalent, and the ratings that actually matter for washdown and outdoors.
6 min read
Compliance5
GAMP 5 for control engineers, without the consultancy
Software categories, the V model, risk based effort, and what actually changes about how you write and test a PLC program.
8 min read
21 CFR Part 11 where the PLC is concerned
Electronic records, electronic signatures, audit trails, and the parts a controller genuinely has to satisfy.
8 min read
ISA-88 in practice: recipes, equipment and the separation between them
The physical and procedural models, why a recipe should not name a valve, and what a phase actually is.
8 min read
ISA-95 levels, and where your data actually has to go
Levels 0 to 4, what belongs at each, and why the level 3 gap is where most integration projects live.
7 min read
ALCOA+ as a design checklist, not an audit finding
Nine words, what each one means for a control system, and the design decisions that satisfy them before anybody asks.
7 min read
AI5
Why ChatGPT writes ladder logic that does not run
General models are good at the shape of PLC code and bad at the parts that matter. Here is what they get wrong, and what has to sit around them.
8 min read
Agentic AI on the plant floor: what is actually running
Every vendor has announced one. A smaller number have deployed. Here is what is genuinely in production, and what the word is doing in those sentences.
10 min read
Running a local model for PLC work
Most OT networks have no internet access, which makes local models the only option rather than a preference.
8 min read
Where AI-written code stops being acceptable
A line worth drawing explicitly, before somebody draws it for you after an incident.
7 min read
The one AI task in automation with no downside
Documentation is the most disliked, most billable and least risky thing to hand to a machine.
6 min read
Practice14
Writing a machine sequence as a state machine
One integer, one rung per state, one rung per transition, and the three rules that keep it debuggable at three in the morning.
8 min read
A method for troubleshooting a PLC that will not run
An order of checks that finds the fault faster than staring at the ladder.
8 min read
Documentation people actually read
Four documents carry almost all the value, and most projects produce them in the wrong order.
7 min read
Version control for PLC code, realistically
Git works better than most engineers expect on some platforms and barely at all on others, and knowing which is which saves an argument.
7 min read
Alarms people act on rather than silence
An alarm system that cries wolf is worse than none, and the failure is almost always in the design rather than the operator.
7 min read
HMI screens that help at three in the morning
The design question is not how it looks on the handover day but whether it helps somebody tired and under pressure.
7 min read
PID tuning without the maths
A method that gets a loop stable in twenty minutes, and the three symptoms that tell you which term to change.
8 min read
Motor control logic that survives a real plant
The rungs beyond start and stop: interlocks, permissives, run feedback and the fault that nobody plans for.
7 min read
Commissioning: the order that finds problems early
A sequence that surfaces the expensive faults while they are still cheap to fix.
8 min read
Security for control systems, without the theatre
A short list of things that genuinely reduce risk on an OT network, and the ones that only look like they do.
7 min read
Naming and structure conventions worth adopting
A convention nobody follows is worse than none. Here is a small set that survives contact with a real project.
6 min read
When scan time matters, and what to do about it
Most programs do not need optimising. Recognising the ones that do, and knowing where the time actually goes.
7 min read
PLC simulators: what each one is actually for
Vendor simulators, virtual factories and browser tools solve different problems, and picking the wrong one wastes weeks.
7 min read
Getting into automation, and what actually gets you hired
The skills that appear in job adverts and the skills that get you through an interview are not quite the same list.
7 min read