Whether you are applying for your first PLC engineering role or a senior controls position, interview questions follow predictable patterns. This guide covers the 50 most commonly asked questions in UK automation engineering interviews, grouped by topic and with model answers you can adapt to your own experience.
At EDWartens UK, we have prepared thousands of engineers for interviews at major automation employers. This list distills the most frequent questions we see across water utilities, FMCG, automotive, pharmaceuticals, and engineering consultancies.
Section 1: PLC Fundamentals (Questions 1–10)
1. What is a PLC and how does it differ from a general-purpose computer? A PLC (Programmable Logic Controller) is a ruggedised industrial computer designed for real-time control of machinery and processes. Unlike PCs, PLCs run deterministic scan cycles, tolerate harsh environments, and prioritise reliability over raw performance.
2. Explain the PLC scan cycle. The scan cycle has four phases: (1) input scan reads all field inputs, (2) program execution evaluates logic from top to bottom, (3) output scan writes to physical outputs, (4) housekeeping handles communication and diagnostics. This cycle repeats continuously, typically every 1–50 ms.
3. What are the five IEC 61131-3 programming languages? Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL, now deprecated), and Sequential Function Chart (SFC).
4. When would you use Structured Text over Ladder Logic? Structured Text is better for complex mathematical algorithms, string manipulation, and loops. Ladder is better for Boolean logic and where maintenance technicians need to read the code.
5. What is the difference between a latching and non-latching output? A non-latching output is only energised while its rung conditions are true. A latching output (using SET/RESET or seal-in contacts) remains ON after the triggering event, until explicitly reset.
6. What is a PLC scan time and why does it matter? Scan time is how long the PLC takes to complete one full cycle. Typical values are 5–50 ms. Critical for fast processes — if scan time exceeds your input signal duration, the PLC may miss events.
7. What is a retentive memory register? Memory that retains its value after a power cycle. Non-retentive memory resets to zero on restart. Important for machine state that must survive power failures.
8. What is the difference between a PLC and a PAC? A PAC (Programmable Automation Controller) extends PLC capabilities with more advanced features: stronger math, motion control, data logging, and web interfaces. Modern S7-1500 and ControlLogix blur the line between PLC and PAC.
9. What are common PLC input types? Discrete/digital (24V DC or 110V AC), analog (0–10V, 4–20 mA), high-speed counter inputs, thermocouple/RTD for temperature, and specialty modules for weighing cells, encoders, etc.
10. Explain sinking vs sourcing inputs. A sinking input provides the current path to ground; a sourcing input provides current to the load. The distinction matters when wiring to sensors: NPN sensors need sourcing inputs, PNP sensors need sinking inputs. European standards favour PNP/sinking.
Section 2: Siemens TIA Portal (Questions 11–20)
11. What is the difference between an OB, FB, FC, and DB in Siemens?
- OB (Organisation Block): Entry point (e.g., OB1 is cyclic main).
- FB (Function Block): Reusable code with persistent memory (via instance DB).
- FC (Function): Reusable code without persistent memory.
- DB (Data Block): Structured data storage.
12. What is an optimised block in TIA Portal? A block with memory layout optimised by the compiler rather than absolute addressing. Optimised blocks are faster and easier to modify but lose direct addressing capabilities.
13. Explain PROFINET IO vs PROFIBUS DP. PROFINET is Ethernet-based (100 Mbps+), supports real-time I/O, motion control, and is the modern standard. PROFIBUS DP runs on RS-485 at up to 12 Mbps and is being phased out for new projects.
14. What is the purpose of the S7-1500 HF (high feature) CPU? HF CPUs offer expanded memory, faster processing, more communication channels, and advanced features like motion control and diagnostic buffer history.
15. How do you handle diagnostic alarms in Siemens? Use OB82 (diagnostic interrupt) and OB85 (CPU redundancy error). Configure hardware diagnostic alarm properties on individual modules. The diagnostic buffer stores the last 3200 events.
Section 3: Allen-Bradley / Rockwell (Questions 16–25)
16. What is the difference between MicroLogix, CompactLogix, and ControlLogix?
- MicroLogix: Small systems, limited I/O (legacy, now superseded by Micro800).
- CompactLogix: Medium systems, rack-mounted modules.
- ControlLogix: Large systems, multi-CPU redundancy, high-performance motion.
17. What is a routine in Studio 5000? A routine is a block of executable code within a program. Main routine calls other routines (LD, ST, FBD, SFC). The scheduled program/task determines execution.
18. What is a periodic task vs a continuous task? A continuous task runs whenever no higher-priority task is running. A periodic task runs at a fixed interval (e.g., every 100 ms) regardless of other activity. Periodic tasks are used for time-critical control loops.
19. What is the difference between Tags, Aliases, and Program Tags?
- Controller tags: Global, visible to all programs.
- Program tags: Local to a specific program.
- Aliases: Friendly names pointing to another tag, often used for I/O.
20. Explain Add-On Instructions (AOIs). AOIs are user-defined function blocks that encapsulate reusable logic with their own parameters and local tags. Similar to Siemens FBs.
Section 4: SCADA & HMI (Questions 26–35)
21. What is SCADA? Supervisory Control and Data Acquisition — a centralised system that monitors and controls multiple PLCs/RTUs, logs historical data, displays operator graphics, and manages alarms.
22. Name three popular SCADA platforms. Siemens WinCC, Rockwell FactoryTalk View SE, Ignition by Inductive Automation, Wonderware/AVEVA System Platform, iFIX by GE.
23. What is a tag database in SCADA? A registry of all addressable data points — PLC addresses, names, engineering units, alarm limits, scaling. The backbone of every SCADA screen, trend, and alarm.
24. Explain alarm prioritisation. Alarms are typically grouped by severity: Critical (immediate action), High (requires attention within minutes), Medium, and Low (informational). Good design prevents alarm flooding during upsets.
25. What makes good HMI design? Clear visual hierarchy, consistent colour coding (grey for normal, red for alarm, yellow for warning), minimal decoration, situational awareness at a glance. Follow ISA 101 or ASM Consortium guidelines.
Section 5: Industrial Communication (Questions 36–40)
26. Compare Modbus TCP vs PROFINET. Modbus TCP is a simple, polling-based, universally supported protocol — great for integration. PROFINET is deterministic, cyclic, and faster — better for real-time motion and I/O. Modbus is easier to troubleshoot; PROFINET needs specialised tools.
27. What is OPC UA? Open Platform Communications Unified Architecture — a modern, platform-independent, secure industrial communication standard replacing legacy OPC Classic. Key for IIoT and Industry 4.0.
28. How does EtherCAT differ from PROFINET? EtherCAT uses "telegram on the fly" — the frame passes through each slave node and is modified in real time. Incredibly low jitter, cycle times under 100 µs. Favoured by Beckhoff and motion-heavy applications.
29. What is the difference between a managed and unmanaged Ethernet switch for industrial use? Managed switches allow VLANs, QoS, port mirroring, SNMP monitoring — essential for industrial networks. Unmanaged switches are plug-and-play but blind to problems.
Section 6: Safety & Functional Safety (Questions 41–45)
30. What is SIL (Safety Integrity Level)? A measure of the risk reduction a safety instrumented function provides. SIL 1 (lowest, 10–100x risk reduction) through SIL 4 (highest, 10,000–100,000x). Most industrial applications are SIL 2 or SIL 3.
31. What is a safety PLC? A PLC designed to meet functional safety standards (IEC 61508, IEC 62061). Features include dual-channel CPU, self-diagnostics, fail-safe outputs, and certified safety function blocks.
32. What is Performance Level (PL)? A machinery safety rating per ISO 13849, from PL a (low) to PL e (highest). PL d and e are typical for most guarding and emergency stops.
33. Explain category 3 vs category 4 in ISO 13849. Both require that a single fault does not lead to loss of the safety function. Category 4 additionally requires that an accumulation of faults does not lead to loss of safety — typically achieved with dual-channel monitored systems.
Section 7: Behavioural & Problem-Solving (Questions 46–50)
34. Walk me through a time you diagnosed a difficult PLC fault. Use STAR: Situation, Task, Action, Result. Example: "A conveyor was intermittently stopping. Using trend data, I identified drops in the drive bus communication every 30 seconds. Root cause was a flaky PROFINET cable crimp. I replaced the cable and added a new diagnostic alarm to catch similar issues earlier."
35. How do you approach a PLC project you have never worked on before? (1) Read the functional spec, (2) study the P&ID, (3) trace I/O lists against the code, (4) review alarm history, (5) identify safety critical paths, (6) ask the current maintenance team about known issues.
36. Tell me about a time you disagreed with a design decision. Focus on constructive disagreement. Example: "The initial design called for local HMIs on each line, but I argued for a centralised SCADA with role-based views. This reduced hardware cost, unified alarm management, and enabled remote support."
37. How do you stay current with automation technology? Reading IEC standards updates, Siemens/Rockwell technical documents, following ISA publications, attending SPS or Automatica trade shows, engaging with communities on EDWartens blog, and hands-on experimentation with new hardware.
38. What is Industry 4.0 and how does it affect your role? Industry 4.0 represents the integration of PLCs, SCADA, IIoT, cloud analytics, AI, and predictive maintenance. A modern automation engineer must understand not just traditional control but also data, cybersecurity, and system integration.
39. Describe your approach to cybersecurity on OT networks. Defence in depth: segmented networks, firewalls between IT and OT, whitelisted traffic, OEM security patches, strong authentication on HMIs, backup strategy, and logging. Follow IEC 62443.
40. Why do you want to work for [employer]? Research the employer's specific projects, technologies, and industry. Tie your answer to their recent wins, your relevant experience, and your long-term career goals.
How EDWartens Prepares Students for Interviews
Our Professional Module includes dedicated interview preparation: mock technical interviews, portfolio development with real PLC projects, behavioural coaching, and access to our network of UK automation employers. Graduates routinely secure offers within 60 days of course completion.
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