Case study
PLC-5 to ControlLogix migration on a fab's process cooling water plant
An obsolete PLC-5 running PCW for a tool bank, replaced over a single 14-hour window with a tested route back to the original system.
The problem
What we found
The process cooling water system feeding a tool bank ran on an Allen Bradley PLC-5 that had been out of production for years. Spares were being bought second-hand from brokers with no provenance, and the site had one remaining processor card in stores.
There was no source code. The last documented change had been made by a contractor who was no longer trading, and the printed schematics in the panel did not match the wiring in three places.
The constraint
The fab could offer one 14-hour window across a planned maintenance weekend. If the plant did not restart within it, tool availability was affected on the Monday.
The approach
How it was done
- 01
As-found survey before anything was quoted
Two days on site recovering the program from the processor, tracing the three wiring discrepancies, photographing the panel and building an I/O list that matched reality rather than the drawings.
- 02
New panel built and proved off-line
A CompactLogix-based panel was built and FAT-tested in the workshop while the plant kept running, with the migrated program simulated against forced I/O.
- 03
Logic reviewed line by line, not converted
Conversion tools produced a starting point, not an answer. Instruction-level behaviour differences were reviewed individually, and the interlock matrix was documented for the first time in the plant's life.
- 04
Rollback proved before the window opened
The PLC-5 panel was left in place and re-connectable within 90 minutes. That route was tested, not assumed, and an abort point was agreed with production at hour nine.
The result
What changed
Changeover completed inside the window with four hours to spare. The plant ran through the following production week without a control-related stop.
The site now holds source code, an as-built I/O list and a documented interlock matrix for a system that previously had none of them.
Next step
Something similar on your site?
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