Topic
Practice
How the work is done well: troubleshooting method, documentation, version control, standards, commissioning, security and the career itself.
18 articles
PLC ladder logic practice problems with solutions you can run
Practise ladder logic on small problems you can test: a seal-in, a one-button toggle, an interlock, timers, a flasher, a counter, a latched fault, hysteresis and an XOR. Write each rung yourself, run it on a simulator and check it against the stated behaviour, not against a picture. The nine problems below each give the solution and the common wrong answer.
11 min readTraffic light ladder logic: a PLC program you can run and test
Build the traffic light as a step machine: an integer holds the current step, one timer measures how long the step has lasted, and a separate block of rungs turns the step number into lamps. Every green is followed by amber and then an all-red clearance before the other road gets green, and a conflict check forces flashing red if two greens ever appear.
9 min readPLC interview questions and answers: what interviewers listen for
Expect three kinds of question: fundamentals (the scan cycle, normally open and closed, sinking and sourcing, 4-20 mA, timers and counters), a rung to write (a start/stop, a toggle, a timed sequence), and a fault to find. Interviewers mark the reasoning more than the definition: what happens scan by scan, what fails safe, and what you check first.
10 min readControlLogix major fault codes: Type 4 Code 20 and the rest
A Logix major fault is reported as a type and a code, such as Type 4 Code 20 (T04:C20): an array subscript too large, or a CONTROL structure's POS or LEN invalid. Type 1 is power-up, 3 I/O, 4 a program instruction, 6 a task watchdog, 7 nonvolatile memory and 11 motion. Read it on the Major Faults tab, fix the cause, then clear it.
9 min readWriting a machine sequence as a state machine
Hold the current state in an integer. Write one rung per state driving that state's outputs, conditioned on the state number. Write one rung per transition, conditioned on the current state and the event, setting the new state number. Never set the state from more than one place per transition, always drive outputs from the state rather than from the transition, and always provide a fault state.
8 min readA method for troubleshooting a PLC that will not run
Work from the outside in: confirm power and the processor mode, then check the physical input at the terminal, then the input in the program, then the logic, then the output in the program, then the output at the terminal. Most faults are found in the first three steps, and starting at the ladder wastes the most time because the ladder is usually right.
8 min readDocumentation people actually read
Four documents carry most of the value: an I/O schedule, a control narrative, the tag list with comments, and a record of what changed and why. The I/O schedule and tag list are mechanical and should be generated. The narrative and the change record are where the engineering judgement lives and cannot be automated.
7 min readVersion control for PLC code, realistically
It depends entirely on whether the platform stores projects as text. Beckhoff TwinCAT files are XML and diff meaningfully. Rockwell L5X exports are XML and work if you export deliberately. Siemens and most others store binary projects, where Git can store versions but cannot show you what changed, which is most of the value.
7 min readAlarms people act on rather than silence
An alarm should require an operator action, and anything that does not should be an event or a log entry instead. The standard failure is alarm flood: hundreds of alarms from one root cause, which trains operators to acknowledge everything without reading. Deadbands, delays and suppression of consequential alarms fix most of it.
7 min readHMI screens that help at three in the morning
Design for the abnormal case, not the demonstration. High-performance HMI practice uses a grey background with colour reserved for abnormal conditions, so anything coloured means something. An overview screen should let an operator see whether the plant is healthy in one glance, with detail one level down.
7 min readPID tuning without the maths
Start with proportional only, increase gain until the loop oscillates steadily, then halve it. Add integral until the offset disappears, slowing it if the loop starts hunting. Leave derivative at zero for most loops, because it amplifies measurement noise and is rarely needed on temperature or level.
8 min readMotor control logic that survives a real plant
Beyond the seal-in, a production motor needs a permissive chain that must be true to start, interlocks that stop it while running, run feedback to confirm it actually started, and a failure-to-start timer. The distinction that matters most is between a permissive, which is checked at start, and an interlock, which is checked continuously.
7 min readCommissioning: the order that finds problems early
Prove things in an order where each step depends only on what is already proven: power and safety circuits first, then I/O point by point with the program stopped, then individual devices in manual, then sequences, then the full cycle. Skipping straight to running the sequence is what turns a wiring fault into two days of debugging logic.
8 min readSecurity for control systems, without the theatre
The measures that matter most are segmentation from the business network, removing default credentials, keeping controllers in RUN rather than REMOTE, and controlling who can physically reach the panel. Most control systems fail on the basics rather than on anything sophisticated, and an internet-exposed HMI is still the most common serious finding.
7 min readNaming and structure conventions worth adopting
The conventions that survive are short: descriptive tag names that read in a rung, a consistent prefix or suffix scheme for signal types, one routine per machine area, and comments that explain intent rather than mechanism. A short standard people follow beats a comprehensive one they abandon in week two.
6 min readWhen scan time matters, and what to do about it
First measure rather than guess: most platforms report scan time and per-task timing. The usual causes are large loops, unnecessary floating point in cyclic code, and communication instructions executing every scan. Moving slow work into a periodic task at a lower rate fixes most problems without touching the logic.
7 min readPLC simulators: what each one is actually for
It depends on what you are trying to prove. Vendor simulators such as PLCSIM and Emulate reproduce their own platform faithfully and require its licence. Factory I/O adds a 3D process to drive. Browser tools need nothing installed and suit learning logic rather than platform specifics. The property that matters most is whether the simulator models the scan honestly.
7 min readGetting into automation, and what actually gets you hired
The reliable route combines electrical fundamentals, one platform learned properly, and evidence you have made something work. Employers weigh demonstrable troubleshooting ability above breadth of platform knowledge, because platforms can be taught in weeks and diagnostic thinking cannot. A simulator plus a small hardware rig is enough to produce that evidence.
7 min read