Control system modernisation without production downtime
In a 24/7 production environment, every unplanned stop is costly. GCG uses three cutover strategies, parallel run, hot standby and rolling cutover, that make PLC migration possible without weeks of production downtime.
A 24/7 factory cannot stand still for weeks
A PLC migration on a continuous production line is technically feasible, but only if the cutover is carefully prepared. Customers who ask for "just a quick switchover" underestimate what goes wrong if the new controller on line 1 does not work and the line has already been down for ten minutes.
GCG uses three proven cutover strategies, depending on your production schedule, the criticality of the machine and the available downtime.
3 strategies for low-risk migration
Parallel run
Old and new controller run simultaneously on the same installation. The new PLC receives the same I/O signal as the existing one. The programme is verified under live production conditions. Switchover takes place once both systems show identical behaviour.
Hot-standby switchover
The new PLC stands ready in hot-standby configuration. At the planned switchover moment, the switch from old to new is made in one action, typically within seconds. The rollback runbook is ready in case the switchover needs to be reversed.
Rolling cutover
The migration is carried out in phases per machine or per line segment. Each phase has its own switchover window (typically a weekend or a planned stop), with validation before the next phase begins.
What is essential before the cutover
Factory Acceptance Test on exact duplicate hardware
The FAT takes place on a fully identical test setup at GCG: same PLC type, same I/O modules, same software version. Not on a "comparable" system.
Complete I/O test with process values
Every I/O point is tested with actual process values, not just simulations. Including all alarm functions, safety interlocks and communication protocols.
Rollback runbook with exact timelines
Documentation of every step in the cutover, including decision moments: "if X does not work within Y minutes, return to old system via step Z". Rollback must be executable without debate during the cutover.
Customer team availability during cutover
A cutover only succeeds if the customer side is present and available: process operators who know the machine, electrical maintenance for physical verifications, and decision authority for go/no-go.
- ✓Sufficient lead time for FAT (minimum 4 weeks)
- ✓Parallel wiring is physically feasible
- ✓Customer team available during cutover
- ✓Rollback scenario defined and tested in advance
- ✗Scope still open at cutover time
- ✗No rollback option planned
- ✗Customer team not present or reachable
- ✗FAT on a "comparable" system instead of exact hardware
From practice
Siemens S7-300 migration on a continuous food processing line. FAT on duplicate hardware at GCG. Cutover on Saturday 23:00, production resumed Monday 03:00 without unplanned downtime.
Allen-Bradley SLC 500 migration on a 3-shift textile line. One weekend cutover per line segment, validation before next phase. Total migration duration: 3 weekends, zero unplanned production stoppages.
Frequently asked questions
What if the scope is not yet fully defined at the time of cutover?
A cutover with an open scope is a recipe for failure. GCG only works with a cutover date once the scope is frozen, the FAT has passed and the rollback runbook is ready. If the scope is still open: close it first, then plan.
What does a parallel run cost in terms of extra hardware?
Temporarily a second PLC set (CPU + I/O) for the duration of the parallel run. This can be rented or included in project costs. The additional verification opportunity outweighs the hardware costs for most customers.
How long does a typical cutover take?
A hot-standby switchover typically takes 2–8 hours. A rolling cutover per segment 1–2 days per phase. The FAT and preparation phase are more decisive for total migration time than the cutover itself.
Plan your control system modernisation without production downtime
GCG combines FAT, cutover planning and rollback runbook in one migration project. We plan around your production schedule.