We Process PLC Files. Here's What the Data Shows.
We process PLC files from industrial facilities worldwide. Over the past year, patterns emerged from that data — consistent findings about program size, documentation quality, instruction distribution, and change management that no one in the industry publishes. This is that data.
About the Dataset
Our analysis covers 500+ ACD and L5X files from Allen-Bradley ControlLogix and CompactLogix controllers. The programs come from manufacturing environments across multiple industries: automotive (31%), food and beverage (24%), semiconductor (18%), oil and gas (14%), and water/wastewater (13%). Files span Studio 5000 versions v21 through v36. All data is anonymized — no facility names, tags, or identifying information is included.
Instruction Distribution: Eight Instructions Drive 85% of All Logic
When we parsed the rungs across all 500+ programs, a clear pattern emerged. Just eight instruction types account for the vast majority of ladder logic. The distribution is consistent across industries and program sizes:
| Instruction | % of Rungs | Purpose |
|---|---|---|
| XIC | 34% | Normally open contact |
| OTE | 22% | Output energize (coil) |
| XIO | 18% | Normally closed contact |
| TON | 5% | Timer on-delay |
| OTL/OTU | 3% | Latch/unlatch |
| CTU | 2% | Count up |
| MOV | 1% | Move/copy value |
| Other 215+ | 15% | Motion, math, comms |
This concentration is significant. Engineers who understand these eight instructions and their interaction patterns can read and troubleshoot the vast majority of the ladder logic they'll encounter in the field.
Program Size and Complexity
PLC programs vary dramatically in scope. Our dataset shows clear patterns by facility type:
- Median Program — 847 rungs, 12 routines, 3 tasks. This is the typical mid-size plant controller.
- Mean Program — 1,340 rungs (skewed by large programs). The average is higher than the median because some industrial systems have extremely complex programs.
- Largest Program — 14,200 rungs, 94 routines (automotive body line with multi-axis motion control and complex interlocking logic).
- Smallest Program — 45 rungs (simple conveyor or motor starter control).
- Add-On Instructions (AOIs) — 34% of programs used at least one AOI. The average program with AOIs contained 8 AOIs. Only 5% of programs had more than 15 AOIs.
Programs from automotive and semiconductor facilities averaged 2,400+ rungs, significantly higher than food and beverage (avg 780 rungs) or water treatment (avg 650 rungs).
The Documentation Gap: 67% Have At Least One Undocumented Routine
This is the finding with the most direct operational impact. Documentation gaps correlate with extended downtime during fault diagnosis. Our analysis of 500+ files reveals a consistent under-documentation problem:
- 67% of programs had at least one routine with zero rung comments. This is not a best practice — it's the norm.
- 11% of programs had documentation on more than 80% of rungs. We consider this threshold adequately documented.
- Average comment coverage across all programs — 23%. This means 77% of rungs in a typical program have no explanation.
- Programs below 30% comment coverage correlated with 3.2x longer diagnosis time in fault events reported through our platform.
Why Documentation Matters for Downtime
When a PLC fault occurs, technicians must first diagnose the fault (hardware issue, bad tag, logic error). But if the program has no comments, they spend hours reading the logic to understand what should happen. Hardware failures average 1–2 hours to repair once diagnosed. Without documentation, diagnosis itself becomes the bottleneck — adding 3–6 extra hours to total downtime. The documentation gap is not a nice-to-have. It directly extends production outages.
Undocumented Changes: Average 12 Per File, Up to 40+ in Active Facilities
When we compared sequential backups of the same program, we discovered a troubling pattern: undocumented modifications. These are changes to rung logic, setpoint values, tag assignments, or instruction parameters with no accompanying note, change date, or technician ID.
- Average per file — 12 undocumented modifications.
- High-change environments — Active production facilities with frequent tuning exceeded 40 undocumented changes per backup cycle.
- Most common silent changes — Timer preset values (38% of undocumented edits), XIC/XIO contact additions (27%), rung enable toggles (19%), tag rename (16%).
- Change history tracking — Only 8% of files had any embedded change history or version notes.
These undocumented changes pose a compliance risk, a safety issue, and a troubleshooting liability. When a production line fails, you cannot determine who changed what, when, or why.
What This Means for Your Operation
Three immediate actions:
- Start documenting now. Even retroactive comments on the 20% most-critical rungs (fault handling, setpoint logic, interlocks) dramatically reduce fault diagnosis time. You don't need 100% coverage to see the benefit.
- Compare before overwriting. Before deploying a new program version, upload both the current and previous backup to our platform and see what changed. This prevents silent mistakes and provides audit trail evidence.
- Measure your coverage. Upload your ACD or L5X file and see your program's rung comment percentage, identify undocumented routines, and benchmark against other programs in your industry.
Frequently Asked Questions
What is the most common instruction in Allen-Bradley ladder logic?
XIC (Examine If Closed) is the most frequent instruction, appearing in approximately 34% of all rungs. It is followed by XIO (Examine If Open) at 18% and OTE (Output Energize) at 22%. Together these three account for nearly three-quarters of all ladder logic instructions in typical programs.How large is a typical PLC program?
Based on our analysis, the median Allen-Bradley ControlLogix program contains 847 rungs across 12 routines. Programs ranged from 45 rungs (simple machine controllers) to 14,200 rungs (multi-axis motion systems with complex interlocking). Programs from automotive and semiconductor facilities averaged 2,400+ rungs.What percentage of PLC programs are poorly documented?
67% of ACD and L5X files we analyzed had at least one routine with zero rung comments. Only 11% had documentation on more than 80% of rungs — a threshold we consider adequately documented. The average comment coverage was 23%, meaning 77% of rungs had no explanation attached.How many undocumented changes are in a typical PLC program?
When comparing sequential backups of the same program, we found an average of 12 undocumented modifications per file — changes to rung logic, setpoint values, or tag assignments with no accompanying note, date, or technician ID. In high-change environments (active production facilities with frequent tuning), this number exceeded 40.What is the most common source of extended PLC downtime?
Poor program documentation. Hardware failures average 1–2 hours to repair once diagnosed. But in programs with no rung comments, technicians spent 3–6 hours reading logic before they could begin repairs. The fault itself was secondary — the documentation gap was the primary driver of extended downtime.