Top 7 PLC Programmer (Siemens) Interview Questions (2026)

Behavioral questions

Past-experience questions. Answer with the STAR method: Situation, Task, Action, Result.

1. 1

   Tell me about a complex project you developed in TIA Portal from concept to commissioning.

   What they're really asking: Full-cycle experience: TIA Portal's integrated approach means the PLC program, HMI, drive parameterization, and network configuration all live in one project. Programmers who've used that integration end-to-end are more efficient than those who've done only the PLC portion.

2. 2

   How do you structure a large TIA Portal project for maintainability across multiple programmers?

   What they're really asking: Project architecture for teams: library management, consistent FB naming and interface conventions, comment standards, and version control with TIA Portal's multiuser project capability. Large projects without these conventions become unmaintainable.

Technical questions

Skill and knowledge checks. Be specific — name tools, tolerances, and methods.

1. 1

   Explain the difference between Function Blocks, Functions, and Organization Blocks in Siemens S7.

   What they're really asking: S7 program structure fundamentals: FBs have instance data blocks and retain state, FCs are stateless and use local temporary memory, OBs are the system's entry points (startup, cyclic scan, interrupt, fault). Wrong block type for the application creates bugs that are hard to find.

   Strong answer:

   Organization Blocks

   OBs are called by the operating system, not by user code. OB1 is the main cyclic scan. OB100 runs at startup. Interrupt OBs run on hardware events. Your program lives in OBs or is called from them.

   Function Blocks

   FBs are the workhorses for device logic: a cylinder control FB, a motor starter FB. Each call to an FB has its own instance DB where state data persists between scans — the cylinder's step counter, a timer, a fault latch. I use FBs for anything that needs to remember state.

   Functions

   FCs are stateless — every call starts fresh, all data is in input/output parameters. I use FCs for calculations, conversions, and utility logic where state doesn't matter. No instance DB means less memory overhead and cleaner interfaces for pure computation.

   The instance DB explanation — that each FB call gets its own memory — is the detail that shows you understand the Siemens memory model rather than just knowing the names.

   Practice answering this question out loud →
2. 2

   What's a Data Block and how do you use Global versus Instance DBs?

   What they're really asking: Siemens data architecture: Global DBs store shared data accessible from any block; Instance DBs store an FB's private data. Programmers who use Global DBs for everything create naming conflicts and maintainability problems on large projects.

3. 3

   Explain the difference between S7-1200 and S7-1500, and how you'd choose between them for a new project.

   What they're really asking: Platform selection: S7-1500 is the full-featured platform with better performance, more instruction set, safety options, and Motion Control; S7-1200 is the compact lower-cost option for smaller machines. Knowing where the capability gaps are is project engineering judgment.

4. 4

   How does Siemens Safety (F-CPU, F-I/O) work and what makes it different from standard programming?

   What they're really asking: Safety fundamentals: F-programs run in a separate safety context alongside the standard program, F-I/O uses dual-channel or diagnostic wiring, and the safety program requires validation and a password-protected signature. Programmers who've done safety projects are a significantly smaller pool.

Situational questions

Hypotheticals that test judgment. Walk through your reasoning step by step.

1. 1

   A customer has an S7-300 system with no TIA Portal project — only the legacy STEP 7 files. They want to migrate to S7-1500. How do you approach it?

   What they're really asking: Legacy migration is a real and common project type. The answer involves using TIA Portal's migration tool as a starting point, understanding what doesn't migrate cleanly (obsolete instructions, hardware config), re-testing all logic, and a commissioning strategy that minimizes production downtime.

How to prepare for a PLC Programmer (Siemens) interview

• 1

  Name your S7 platform experience specifically

  S7-300, S7-400, S7-1200, S7-1500, Safety F-CPU — and the TIA Portal version. The differences are real and interviewers in Siemens shops will probe them.

• 2

  WinCC experience completes the package

  WinCC Basic/Comfort for panel HMIs, WinCC Advanced/Professional for PC-based SCADA — the TIA Portal integration makes the combined skill set significantly more valuable than PLC-only.

• 3

  European machinery exposure is a career differentiator

  Siemens dominates European machine controls. Experience with German, Italian, or other European OEM equipment — along with the ability to read German technical documentation with translation tools — opens doors in automotive and specialty machinery.

• 4

  Ask about their legacy installed base

  A plant with 40 S7-300 machines running STEP 7 is a different role than a greenfield S7-1500 shop. Legacy support and migration work versus new development shapes your day significantly.

Siemens PLC programmers occupy a distinct niche in North American manufacturing — less common than Allen-Bradley specialists but in high demand at automotive, chemical, food and beverage, and European OEM-heavy plants. The combination of Siemens PLC and WinCC SCADA experience is particularly scarce and well-compensated.