PLC Programming

Most undocumented PLC code was written in good faith twenty years ago by an engineer who has long since retired. The plant runs — but no one alive can confidently change it. Our PLC programming service exists to solve exactly that problem: write maintainable, peer-reviewed, version-controlled code today, so the next engineer in (whether ours or yours) can pick it up in minutes rather than days.

Every project follows the same workflow: capture intent in a signed-off Functional Design Specification, develop against our in-house IEC 61131-3 coding standard, peer code-review every commit, run a witnessed Factory Acceptance Test, then commission and Site Acceptance Test on the live plant.

We program every major PLC platform in the market: Siemens (TIA Portal, Step 7, Climatix BMS controllers), Allen-Bradley (Studio 5000 for CompactLogix and ControlLogix, RSLogix 500 for SLC-500 and MicroLogix), Mitsubishi (GX Works2 / GX Works3 for FX, Q-Series and iQ-R), ABB (AC500 in Automation Builder) and Schneider (EcoStruxure Control Expert for Modicon).

All five IEC 61131-3 languages are in active use depending on the task: Ladder Diagram for discrete logic and machine interlocks; Structured Text for maths, recipe handling and looping operations; Function Block Diagram for reusable libraries; Sequential Function Chart for batch and state-machine logic; Instruction List where legacy code requires it. We default to Ladder + Structured Text + FBD for new projects, with SFC reserved for true sequential processes.

Safety functions are designed and validated to BS EN ISO 13849-1 or IEC 62061 as appropriate, with safety-rated PLCs — Siemens Safety Advanced (F-CPU), Rockwell GuardLogix, Pilz PNOZmulti or Sick Flexi Soft — depending on the host platform. Each safety function carries a documented Safety Functional Requirement Specification (SFRS), a SISTEMA or PAScal calculation evidencing the required Performance Level or SIL, and a signed safety validation report at handover.

We also handle the wider machinery safety package — risk assessment, safe operating procedures, lock-off / tag-out documentation and operator training — so the client EHS team has everything they need for HSE inspection.

PLC programming rarely lives in isolation. Every project includes the HMI / SCADA layer developed against the same tag list as the PLC — Siemens WinCC and Comfort Panels, Rockwell FactoryTalk View Studio (ME / SE), Mitsubishi GOT, ABB Panel Builder, Ignition, AVEVA / Wonderware InTouch and Citect. Graphics follow High-Performance HMI principles (ISA-101) — neutral palette, alarm-by-exception and operator-task-focused layouts.

Industrial networking is engineered as part of the same scope: Profinet, EtherNet/IP, Modbus TCP, Profibus and OPC UA, with managed industrial Ethernet (Stratix, Scalance, Hirschmann) and segmented OT VLANs. Where remote support is required after handover, we configure secure site-to-site VPN with MFA and audited session logging.

Every PLC and HMI program lives in a Git-backed repository — TIA Portal Multiuser server for Siemens, FactoryTalk AssetCentre for Rockwell, external Git for cross-platform projects. Every change is committed with a meaningful message, peer code-reviewed against our coding standard and tagged at each release. The client receives a complete copy of the repository at handover, plus signed-off versioned PDFs of the program for offline reference.

Witnessed FAT against the original FDS is run before any site mobilisation. On site, loop checks and point-to-point commissioning precede a witnessed SAT, after which the program is locked, the O&M pack is issued and operator and maintenance training is delivered. PPM contracts after handover include 4-hour critical SLA, remote diagnostics and an annual code review against the latest version of our standard.