Obsolete PLCs fail in clusters: the same batch of capacitors ages across your fleet. Maintenance asks whether to buy the last modules on eBay or accelerate retrofit. There is no universal answer, but there is a disciplined way to decide without panic buying.
Stock when
- Retrofit is funded but more than twelve months away and production cannot tolerate a single point of failure.
- Verified backup images exist and spare hardware has been test-loaded.
- Critical modules are interchangeable across multiple lines, amortising inventory cost.
Do not stock when
Spares sit on shelf without firmware compatibility checks, or nobody remembers which image matches which CPU. That is expensive scrap, not resilience. Likewise, buying unknown-refurb modules without burn-in testing invites weekend surprises.
Summary
Spares buy time; they do not buy strategy. Pair inventory decisions with a dated retrofit gate and documented risk owner.