When outsourcing PLC programming, ask for six document packages: the requirements and functional design specification; I/O, tag, and interface schedule; editable PLC project and code documentation; test and acceptance records; cybersecurity, access, backup, and recovery arrangements; and the as-built handover and change record. Together, they help you define the intended result, review what was built, and maintain it after the contractor leaves. What documents should I ask for when outsourcing PLC programming? Use this checklist as a starting point, then tailor it to your PLC platform, process risk, contract scope, owner standards, and jurisdiction. It is a practical contracting checklist—not a universal legal list or a complete set mandated by IEC.
1. Requirements and functional design specification
Ask the contractor to document the agreed process behavior before programming begins. This is the design basis you can use to decide whether the delivered work matches the requested outcome.
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- Operating modes and the conditions for entering or leaving each mode.
- Sequences, permissives, interlocks, alarms, and expected responses to faults.
- Assumptions, exclusions, and owner-supplied information or equipment.
- How changes to the agreed requirements will be reviewed and approved.
Keep this separate from the code handover: the specification describes what the system is meant to do; the editable project and its documentation show what was programmed. An Irish Water technical specification offers an owner-specific example of requiring an application or software design specification in the handover package: Irish Water WW-1 technical specification.
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2. I/O, tag, and interface schedule
Request a project-specific schedule that identifies the signals and the boundaries between the PLC work and other equipment or contractors. Agree the file format, naming conventions, and revision process before the work starts.
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- Signal or tag name and a plain-language description.
- Field device and PLC channel or address, where applicable.
- Signal type, units, scaling, and relevant normal or fault states.
- Communications interfaces, connected systems, and responsibility boundaries.
- Revision status and a way to trace approved changes.
There is no single schedule format established by the cited project-documentation guidance. Define the columns and level of detail that suit your platform and project rather than assuming the contractor’s preferred spreadsheet is a universal standard. General project documentation and industrial cybersecurity guidance are available in the NATO ENSEC COE industrial control systems cybersecurity guide and the ISA ISA/IEC 62443 series overview.
3. Editable PLC project and readable code documentation
Specify delivery of the native, editable project files needed to understand and maintain the installation—not only a compiled file, printout, or password-protected export. Confirm the required software, platform, and version, and whether licenses or other dependencies are needed to open the project.
- Native project files and relevant software diagrams or listings.
- Comments and explanations for symbols, tags, routines, and non-obvious logic.
- Controller, firmware, programming-software, and project-version details.
- Instructions for restoring a known-good project version and any required dependencies.
- Contract terms covering file ownership, access, permitted use, and retention.
Irish Water’s technical specification says PLC software documentation should allow a competent person to understand and follow the program, and identifies a software design specification and documented diagrams or listings as handover materials. Treat it as an example of an owner’s requirement, not a rule that applies everywhere: Irish Water WW-1 technical specification.
4. Test and acceptance package
Put the evidence required for acceptance into the scope. Ask for test procedures or cases, completed results, records of deviations and open issues, and acceptance documentation. Tie tests to the agreed requirements so you can see what was checked and what remains unresolved.
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- State whether factory testing, site testing, or both apply to this project.
- Define who witnesses or approves tests and how failed tests are handled.
- Record test conditions, results, deviations, corrective actions, and retests.
- Identify any items deferred to commissioning or accepted with an agreed action.
Do not assume every PLC job requires the same FAT/SAT package. IEC 62443-2-4:2023 covers security processes that service providers can offer during integration and maintenance, but it does not establish a universal testing package for every programming contract. Its scope and provision for adapting profiles are described in the IEC 62443-2-4:2023 catalog entry.
5. Cybersecurity, access, backup, and recovery arrangements
Write down who is responsible for each security-related task during the project and after handover. Security is shared between the asset owner and service provider; do not assume that hiring an integrator transfers all operational security responsibility.
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- Who creates, approves, controls, and removes user accounts and remote access.
- How credentials are transferred securely and which party retains them.
- What gets backed up, where the backups are stored, who can access them, and how restoration is verified.
- How program changes are authorized, recorded, reviewed, and rolled back.
- How the parties will handle legacy equipment or other constraints, including any compensating measures.
IEC 62443-2-4:2023 addresses security processes a service provider can offer for integration and maintenance; requirements can be profiled to suit an environment. IEC 62443-2-1:2024 addresses asset-owner policies and procedures for operating industrial automation and control systems, and recognizes that long-lived or legacy systems may require a subset of requirements or compensating measures. See the IEC 62443-2-4:2023 catalog entry and the IEC 62443-2-1:2024 catalog entry. The ISA overview also frames the series around lifecycle and shared responsibility: ISA/IEC 62443 series overview. Backups and source-code control are also covered as industrial cybersecurity program topics in the NATO ENSEC COE guide.
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At closeout, request a package that reflects the controller and configuration actually left in service—not an earlier design snapshot. Use it to establish what is running and what differs from the original scope.
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- The final editable project, identified by version and matched to the installed controller configuration.
- A record of approved changes, deviations, and outstanding issues.
- Operator or maintainer notes needed for routine support and troubleshooting.
- Backup and recovery information, with the responsible party and storage location identified.
- Contract terms for file formats, ownership, access, and retention.
Owner requirements vary: the Irish Water specification is one example of requested software documentation, while IEC guidance addresses service-provider and asset-owner security processes rather than imposing one global handover format. Agree the closeout contents in the contract and tailor them to the project’s risks and scope: Irish Water WW-1 technical specification, IEC 62443-2-4:2023, and IEC 62443-2-1:2024.
How to evaluate a contractor’s document offer
Compare proposals on whether their deliverables are concrete, reviewable, and usable by your team—not just on whether a list of filenames appears in the quote.
- Completeness: Does the scope cover all six packages, with any omissions explicitly identified?
- Ownership and access: Will you receive editable files in formats your team can open and maintain?
- Traceability: Can you connect requirements to tests, results, deviations, and the final installed version?
- Compatibility: Are platform, firmware, software version, and dependencies stated?
- Recovery: Is backup and restore responsibility clear, including how a usable recovery is confirmed?
- Security: Are account, remote-access, credential, and change-control responsibilities assigned between owner and provider?
- Change control: Will approved changes and unresolved deviations be captured in the as-built record?
These are procurement evaluation criteria, not published performance statistics. The right level of detail depends on the process risk, platform, scope, owner standards, and jurisdiction.
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