Proprietary and third-party PLC code conversion utilities typically reproduce about 95% of legacy ladder and statement-list logic into a target IEC 61131-3 environment, leaving a residual 5% that still needs an engineer to walk the rung comments, timer/coil datatypes, and I/O mapping line by line [S3][S5].
That 95% figure is consistent across both vendor utilities (for example Curelom Engineering's cross-platform converter) and practitioner reports dating back more than a decade, which suggests the residual error band is structural rather than a problem any single tool has solved [S3][S5]. For plants running Allen-Bradley SLC 500, Siemens S5, or Modicon 984 hardware, the conversion is rarely the bottleneck; the bottleneck is verifying the translated code against a control narrative that may itself be missing [S4].
Why 95% is the ceiling, not the floor
The 5% gap is not random noise; it concentrates in four recurring patterns: address-based tag references that do not map cleanly to a tag-based target, vendor-proprietary function blocks without an IEC 61131-3 equivalent, undocumented patches added over years of operation, and sequencing or interlock logic that was originally written without a written specification [S1][S3]. IEC 61131-3 (the standard governing PLC programming languages, including ladder, function block, structured text, instruction list, and sequential function chart) gives the converter a stable target grammar, but it does not erase dialect differences between vendors [S1].
Water and wastewater plants are the most exposed to this gap because the sector has historically tolerated looser change management than pharmaceutical or amusement-park operations, so a "lift and shift" conversion often inherits undocumented field fixes rather than the as-designed logic [S1]. When original documentation is missing, even a 99% translation can ship a 100% bug. For an overview of how a PLC is structured, the IEC 61131-3 language set is the reference frame most modern converters target.
What conversion tools actually translate well
Discrete I/O rungs, simple seal-in circuits, basic timer/counter logic, and standard move/compare instructions convert with near-fidelity across Allen-Bradley, Siemens, and Modicon dialects, which is why the headline 95% number holds up in field trials [S3][S5]. Tag-based source code (any program written after roughly 2005 on platforms that adopted tag names instead of fixed addresses) converts better than address-based legacy code, because symbolic names survive the trip even when the underlying memory map changes [S1].
Structured text and function block diagrams move between platforms with the least manual rework, especially when the source already conforms to IEC 61131-3 second or third edition conventions [S1]. A PLC control program built on standard function blocks is the easiest candidate for any converter; one full of vendor-specific motion, recipe, or process instructions is the hardest.
Where conversion tools fail, and what to budget for
Conversion tools underperform on five specific failure classes: proprietary instruction set extensions (for example Siemens S5's FB/DB organisation, Allen-Bradley SLC 500's indirect addressing modes), analog scaling blocks with embedded calibration constants, communication blocks tied to a specific protocol or hardware revision, safety-related logic subject to functional-safety re-certification, and any code that was hand-patched to compensate for a field I/O wiring quirk [S3][S4]. Safety PLCs are a special case because the target program normally has to be re-validated against the original safety case, which a translator cannot do on its own; see the safety PLC reference for the re-certification workflow.
Practitioner experience indicates that software conversion utilities can typically handle only about 95% of legacy PLC code, leaving roughly 5% that must still be combed through by hand to verify and resolve issues [S5], and this manual share can be larger in codebases burdened by undocumented patches and unreliable documentation [S1]. For tooling context, a typical code reader or static analyser is used during this remediation phase to diff the translated logic against the original.
Selection criteria: lift-and-shift converter vs. re-engineering
Choose a conversion tool (lift-and-shift) when the legacy program is well documented, the target hardware stays within the same vendor family, and the control philosophy does not need to change; expect 6 to 10 weeks of effort for a mid-size line and roughly 5% manual rework on top [S3]. Choose re-engineering when the legacy code is undocumented, when safety re-certification is required, or when the migration is also a chance to add IIoT, OPC UA, or MQTT connectivity that the old platform cannot speak [S4][S7].
For a mid-size discrete line, a lift-and-shift conversion typically lands in the 6 to 10 week band, while a re-engineered migration that includes functional re-specification, safety re-validation, and SCADA/HMI rebuilds runs 12 to 16 weeks; the PLC migration timeline for a mid-size line breakdown tracks the same spread. Low-code integration layers (for example Shoplogix-style no-code bridges) are a third path, useful when the goal is data extraction from a legacy PLC rather than replacement of the controller itself [S6][S7].
Reliability and the hidden costs of skipping verification
Legacy Allen-Bradley SLC 500 and Siemens S5 fleets are widely cited as the most common migration candidates in 2025 to 2026 field surveys, with the SLC 500 still installed in thousands of plants long after its formal end-of-life [S4]. Skipping the 5% manual verification on those fleets correlates with unplanned downtime events, because the residual errors are typically concentrated in I/O mapping, timer preset values, and undocumented interlocks that only surface under fault conditions [S8].
Industrial automation reliability guidance from 2026 attributes a meaningful share of brownfield downtime to legacy PLC obsolescence rather than to the replacement hardware itself, which makes the verification step (not the converter) the real reliability lever [S4][S8]. Conversion tools are reliable; the process around them is what determines whether the migrated program is reliable.
Trackable signals for the next planning cycle: end-of-life announcements from the major PLC OEMs on remaining SLC 500 and S5 spares, and the publication of any IEC 61131-3 fourth-edition guidance that tightens the rules for cross-vendor code portability.