The starting point
Manual controls and relay logic are reliable — until they are not. This facility had been operating the production line for over a decade with no significant electrical changes. The relay logic worked, the operators knew its quirks, and nobody wanted to touch it.
The trigger for change was a combination of reliability concerns (aging relay panels, burnt contacts, intermittent faults that were difficult to trace) and a desire to capture production data that was previously tracked by hand at the end of each shift.
Design and scope
The project scope covered the full control system from scratch:
- I/O list developed from physical inspection of all field devices
- Electrical design for new panel, including cable schedules and termination diagrams
- RPEQ engineering package for the new installation
- CompactLogix program written in structured text and ladder, with all sequences, interlocks, and E-stop logic
- FactoryTalk View ME screens for line status, fault display, and production counters
- Loop test documentation and commissioning records
The relay logic was left in place until the new system was fully commissioned and tested — operators ran the line on the new PLC while the old panel remained available as a fallback during the first week.
Documentation
A key deliverable on this project was documentation the maintenance team could actually use. Every PLC routine is commented. The HMI fault list maps fault codes to plain-language descriptions and recommended first-response actions. The as-built drawings reflect what was installed, not the original design.
The maintenance team received a one-day handover covering the PLC program structure, HMI navigation, and how to interpret the commissioning records.