PLC Program History — Change Tracking & Audit Trail
Track Every Change
to Your Machine
Compare two versions of any ACD or L5X file and see exactly what changed: added rungs, deleted outputs, modified setpoints, renamed tags. Build an audit trail and detect undocumented changes that cause mysterious machine faults.
·PLC Company Engineering
- Undocumented PLC program changes are the leading cause of mysterious machine faults — the modification was made, but no one knows when, why, or by whom.
- Comparing two versions of an ACD or L5X file reveals added rungs, deleted outputs, changed setpoints, and renamed tags — changes invisible to production staff.
- Regulatory standards (ISO 9001, FDA 21 CFR Part 11, OSHA PSM) require documented records of all program modifications to safety-critical systems.
- plc.company maintains an audit trail of every upload and provides side-by-side program comparison across any two versions of your file.
Why Machine History Gets Lost
The contractor fix
A controls contractor comes on-site to resolve a production issue. They modify 3 rungs, adjust 2 timer presets, and leave. The machine runs. No change order is filed. Two years later, a different contractor opens the backup from before that visit — and creates a new fault.
The verbal approval
A production supervisor tells a maintenance tech to "bump the reject counter up a bit." The tech enters the value, the machine runs better, nobody writes it down. Three months later the tech moves to a different facility. The change is orphaned.
Version numbering drift
The program archive has 12 files named "Filler_Line3_v2_FINAL_CURRENT_USE_THIS.ACD." Nobody is sure which is actually on the controller. Loading the wrong one takes down the line.
No comparison tool
Even when multiple versions exist, comparing them requires Studio 5000 — which most maintenance staff don't have access to. Without a comparison, there is no way to know what changed between any two versions.
What Gets Changed Without Documentation
Common undocumented modifications that cause latent faults.
Timer and counter presets
The most commonly changed values in an undocumented manner. A 5-second fill timer becomes 4.8 seconds "to improve throughput." No one knows this happened until a downstream quality issue surfaces months later.
Safety interlock bypasses
Temporary bypasses added for troubleshooting ("just to test") that never get removed. A bypassed guard door interlock or light curtain remains in production logic indefinitely.
Alarm setpoints
High/low limit comparators adjusted to suppress nuisance alarms. Over time, the process moves outside its original designed range while alarms remain silent.
Deleted rungs
Diagnostic rungs, maintenance flags, and HMI communication blocks removed to "clean up" the program. The HMI now shows stale values or blank screens.
Regulatory Requirements for Change Documentation
Standards that govern PLC program change management and audit trails.
Standards That Require Change Logs
- ISO 9001:2015 — design and development changes documented
- FDA 21 CFR Part 11 — electronic records integrity for pharma
- OSHA PSM 29 CFR 1910.119 — management of change for hazardous processes
- IEC 62443 — industrial cybersecurity change management
- NRC 10 CFR 50 — nuclear facility software changes
What Auditors Look For
- Date and time of each modification
- Identity of the person who made the change
- Description of what was changed and why
- Before/after values for any modified parameter
- Review and approval record for safety-critical changes
How to Compare Two Program Versions
A four-step process to detect all changes between any two PLC program files.
Upload Both Versions
Upload both versions of your ACD or L5X file to plc.company — as separate project files in the same account.
Open Program Compare
Open the Program Compare tab — the tool diffs the two versions and categorizes changes by type: added rungs, deleted rungs, modified instructions, changed tag values.
Review Each Change
Review each change — for each difference, the side-by-side view shows the exact rung before and after, with instruction-level highlighting.
Export the Report
Export the comparison report — generate a PDF or HTML change log showing every difference between versions, suitable for audit records or change order documentation.
Building a Program Change Audit Trail
Create traceable change records as programs move through your facility.
Every upload is timestamped
When you upload a program file to plc.company, the upload is logged with date, time, and user — creating an automatic record of when each version entered your documentation system.
Annotate changes before they leave your head
The notes feature lets you attach comments to specific rungs, tags, or routines while the change is fresh — before the contractor leaves the site.
Compare any two versions
The Program Comparison tab accepts any two uploaded versions and generates a full diff — useful for audit responses, root cause investigations, and contractor verification.
Export for CMMS or ERP
The audit trail and change log are exportable to PDF and HTML for import into your CMMS work order system or ERP maintenance records.
Related Resources
Frequently Asked Questions
How do I track changes to my Allen-Bradley PLC program?
The most reliable method is maintaining dated file archives — a new .ACD or .L5X export after every change, named with a date and change description. For formal audit requirements, plc.company provides an upload audit trail and program comparison tool that generates documented change reports from any two versions of your file.
Can I compare two versions of an ACD file?
Yes. Upload both files to plc.company and use the Program Compare tab to generate a side-by-side diff. The comparison identifies added and deleted rungs, changed instruction parameters, modified tag values, and renamed routines — exported as a PDF change log for audit records.
What is program version control for PLCs?
PLC version control is the practice of saving dated, labeled copies of your program after each modification — similar to source control in software development. The minimum viable approach is a shared network folder with dated files. More robust approaches use dedicated tools or platforms that timestamp uploads, track who made each change, and enable comparison between versions.
What regulations require PLC program change documentation?
ISO 9001:2015 requires documented design changes. OSHA PSM (29 CFR 1910.119) requires management of change records for safety systems involving hazardous chemicals. FDA 21 CFR Part 11 requires electronic records integrity for pharmaceutical and medical device manufacturers. Each standard has specific requirements for what must be recorded and how long records must be retained.
How do I find undocumented changes in a PLC program?
Compare the controller's current online program to your most recent backup. In Studio 5000, use Controller > Compare to run an online vs. file comparison. Without Studio 5000, upload both versions to plc.company and use the Program Compare tab. Differences in rung logic, tag values, or timer presets that appear in the online version but not the backup represent undocumented changes.
Track Every Change to Your Program
Compare versions, detect undocumented changes, maintain audit trail.