What Does a PLC Programmer Do? Roles, Skills and Toolchains Explained

A PLC (Programmable Logic Controller) programmer designs, develops and commissions the software that runs automated machinery and industrial processes. They sit at the intersection of electrical engineering, software development and process control, translating engineering requirements into reliable, deterministic control logic that keeps production lines, utilities and critical infrastructure running safely. The role spans everything from writing ladder logic for a simple conveyor system to architecting multi-PLC solutions across large-scale process plant with full SCADA integration. Understanding what this role actually involves, both technically and operationally, matters whether you are an engineer considering the specialism or a hiring manager trying to scope a vacancy accurately.

Core Responsibilities of a PLC Programmer

The day-to-day work of a PLC programmer is broader than simply writing code. On most projects, the role covers the full controls engineering lifecycle, from interpreting functional design specifications through to site acceptance testing and handover documentation.

Typical responsibilities include:

• Developing control logic in one or more of the five IEC 61131-3 languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL) and Sequential Function Chart (SFC)
• Configuring hardware: setting up CPU modules, I/O cards, communication modules and safety PLCs within the vendor environment
• Integrating PLCs with SCADA and HMI systems such as Ignition, Wonderware/AVEVA System Platform, Siemens WinCC or Schneider Citect
• Commissioning on site, which involves loop checking, fault finding and tuning control loops
• Writing and executing Factory Acceptance Tests (FAT) and Site Acceptance Tests (SAT)
• Producing and maintaining technical documentation including cause-and-effect matrices, I/O schedules and as-built drawings
• Supporting ongoing maintenance, breakdowns and modifications post-handover

In regulated industries, programmers also interface with functional safety requirements. Where a system falls under IEC 61511 (process industry safety instrumented systems) or IEC 62061 (machinery safety), the PLC programmer may work alongside a functional safety engineer or hold a TÜV Functional Safety Engineer certificate themselves. You can read more about the overlap between controls engineering and functional safety in the Vertech Group article on functional safety recruitment.

PLC Hardware Platforms and Vendor Ecosystems

A PLC programmer's skill set is often shaped by the hardware platforms they have worked with. Each major vendor has its own integrated development environment, communication stack and hardware architecture, so platform experience matters significantly in practice.

Siemens

Siemens is the dominant platform across UK and European industry. The S7-1500 series is the current flagship, programmed via TIA Portal (Totally Integrated Automation Portal), which also covers HMI configuration, drive parameterisation and network engineering within a single project framework. Older S7-300 and S7-400 hardware programmed via STEP 7 (Classic) remains widespread in legacy plant. Siemens uses Profinet as its primary Ethernet-based fieldbus and Profibus for legacy installations. If you are working with Siemens tooling, the Vertech Group article on Siemens TIA Portal provides a detailed technical introduction.

Rockwell Automation (Allen-Bradley)

Rockwell's ControlLogix and CompactLogix families are programmed using Studio 5000 Logix Designer. The platform is heavily adopted in the food and beverage, automotive and life sciences sectors. EtherNet/IP is the primary protocol, built on the Common Industrial Protocol (CIP). DeviceNet and ControlNet appear in older installations. Rockwell also has its own safety PLC range, the GuardLogix, for SIL-rated applications.

Beckhoff

Beckhoff takes a software-first approach. Its CX series of PC-based controllers run TwinCAT 3, which implements IEC 61131-3 within Visual Studio and also supports motion control and C/C++ integration. EtherCAT is Beckhoff's real-time fieldbus, offering high-speed deterministic communication particularly suited to coordinated motion applications.

Other Platforms

• Schneider Electric Modicon (M340, M580, Quantum): programmed via EcoStruxure Control Expert (formerly Unity Pro), common in utilities and infrastructure
• Mitsubishi MELSEC (iQ-R, iQ-F series): widely used in Asian supply chains and UK automotive; programmed via GX Works3
• ABB AC500: often found in power generation and marine applications; programmed via Automation Builder

Programmers who can work across multiple vendor platforms are more versatile, particularly in system integrator roles where projects vary by client and sector.

Communication Protocols and Industrial Networking

A competent PLC programmer needs a working understanding of industrial communication protocols, because integrating field devices, drives, instruments and supervisory systems is a core part of the job.

Commonly encountered protocols include:

• Profinet: Siemens-led Ethernet standard, supports real-time and isochronous communication
• Profibus DP/PA: legacy serial fieldbus, still prevalent in process plant
• EtherNet/IP: Rockwell and Cisco-backed, uses CIP over standard Ethernet
• Modbus TCP/RTU: older but ubiquitous, especially for instrument integration
• HART: used for smart instrument diagnostics over 4-20 mA loops
• EtherCAT: Beckhoff's real-time Ethernet, particularly suited to high-speed motion
• OPC-UA: vendor-neutral, platform-independent data exchange standard, increasingly important for MES and cloud integration under ISA-95 architectures

IEC 62443 is the relevant cybersecurity standard for industrial automation and control systems. As OT/IT convergence accelerates, PLC programmers working on networked systems are increasingly expected to understand network segmentation, secure remote access and patch management principles within the IEC 62443 framework.

Standards, Safety and Regulated Environments

The standards landscape shapes how PLC programmers work, particularly in sectors with safety or regulatory obligations.

IEC 61131-3

This is the foundational programming standard for PLCs. It defines the five programming languages mentioned above and establishes common concepts around programme organisation units (POUs), function blocks, data types and execution models. Most modern PLC environments are IEC 61131-3 compliant, though vendor implementations vary in strictness.

IEC 61511 and Functional Safety

In the process industries (oil and gas, chemicals, water treatment), safety instrumented systems (SIS) must meet IEC 61511. This standard defines how Safety Instrumented Functions (SIFs) are specified, designed, validated and managed across the safety lifecycle. PLC programmers working in this space typically work with dedicated safety PLCs such as the Siemens S7-300F/S7-1500F, Rockwell GuardLogix or Schneider Modicon Safety. Understanding SIL (Safety Integrity Level) verification, proof testing intervals and the separation of safety and basic process control systems (BPCS) is essential.

ISA-88 and ISA-95

In batch manufacturing (pharmaceuticals, food production), ISA-88 defines a procedural control model that shapes how PLC programmers structure their code. ISA-95 defines the integration between the control layer and enterprise systems (MES, ERP), which becomes relevant when PLC programmers are involved in data historian or OPC-UA integration work.

Career Paths and Specialisations

PLC programming is not a single fixed role. It sits within a broader controls and automation discipline, and practitioners typically move in one of several directions as they progress.

Common career development paths:

1. Junior PLC Programmer: Typically focussed on a single vendor platform, working under supervision on modifications and smaller projects
2. Controls Engineer / Senior PLC Programmer: Full project ownership, multi-vendor capability, FAT and SAT responsibility, some customer-facing work
3. Lead Controls Engineer / Controls Architect: System-level design, specification writing, team leadership, functional safety oversight
4. Automation Project Manager: Transition into project delivery, managing controls scope within larger capital projects
5. OT Cybersecurity Specialist: Specialising in IEC 62443 implementation for industrial networks, a growing demand area

Some programmers remain deeply technical and become recognised specialists in a particular vendor ecosystem, a particular process type (batch, continuous, discrete), or a regulated sector such as nuclear, pharmaceuticals or defence.

What Hiring Managers Should Look for in a PLC Programmer

For engineering leaders building a controls team, assessing a PLC programmer's capability requires looking beyond a vendor list on a CV. The following questions tend to reveal genuine depth:

• Can they describe a project where they took a control system from functional specification to SAT?
• Have they worked under a functional safety standard (IEC 61511, IEC 62061) and do they understand the documentation requirements?
• Which SCADA or HMI platforms have they configured and what does their integration experience look like?
• Can they fault find and modify legacy code, not just write new systems?
• Do they have site commissioning experience and are they comfortable with the physical environment of industrial plant?

The breadth of the controls market means that a specialist with deep Siemens TIA Portal experience may be a poor fit for a Beckhoff motion application, and vice versa. Platform specificity matters far more than in many other software disciplines, so scoping the hardware environment accurately before writing a job description saves significant time.

For context on how long it typically takes to hire specialist engineers in the UK, the Vertech Group article on time to hire benchmarks covers relevant data for technical roles across engineering disciplines.

How Vertech Group Supports Industrial Automation Hiring

Vertech Group recruits across two core engineering practice areas: Embedded and Product Engineering, and Industrial Automation and Controls. The controls practice covers PLC programming, SCADA engineering, DCS engineering, functional safety and OT/IT integration roles across sectors including manufacturing, process industries, infrastructure, defence and energy.

For organisations hiring PLC programmers, the challenge is usually one of specificity: the candidate pool is relatively small, platform experience is not transferable in the way general software skills are, and experienced site commissioners are in short supply. Vertech Group works with both systems integrators and end-user organisations across the UK, understanding the technical and commercial context of each role rather than applying a generic search process.

If you are building a controls team or looking for your next automation role, get in touch with the Vertech Group team to discuss how we can help.