When I first started managing Siemens-based production lines back in 2014, I assumed that the newest PLC hardware (like, say, a S7-1500 over a S7-1200) was always the right upgrade path. I thought newer meant more reliable, faster cycles, and lower long-term cost. After nine years and a handful of expensive mistakes, I've come to believe the opposite. In my opinion, for 80% of maintenance and operations teams out there, a hardware upgrade is the wrong first move.
I'm not saying hardware is never the answer. But the way I see it, the push to upgrade to the 'latest generation' is often a vendor-driven solution looking for a problem that doesn't exist at your site. Let me explain why.
In September 2022, one of my team leads argued we needed to replace 12 S7-1200 PLCs on a packaging line (think labels, wrapping, and conveyor logic). His reasoning was that the CPU firmware was three major revisions behind, and a parts distributor had hinted that support was being 'wound down.' We panicked, quoted the project, and approved a $6,200 budget for new hardware, programming, and re-commissioning man-hours.
Here's what actually happened: The line went down for 3 extra days because the new CPU's I/O mapping was slightly different. We lost $2,100 in downtime. One junior programmer fried two terminals by mis-wiring the 24V supply (that was a $900 oopsie). In the end, we had a perfectly new 1500-series controller running a program that *used to work just fine on the old hardware*. We didn't gain a single cycle-second of performance. We just bought ourselves a migraine.
The lesson? The 'end of life' threat was a paper tiger. The old PLCs were working perfectly. We replaced them for *comfort*, not for *need*.
So, if you shouldn't buy the new PLC box, what should you spend your budget on? From my perspective, there are three areas that deliver ten times the ROI of a hardware upgrade. These aren't theoretical—I've used them all on actual client sites (including some big food & beverage and pharma plants).
I know someone is thinking: 'But my S7-300 is slow. I need a faster processor.' Don't hold me to this, but in my experience, the bottleneck is rarely the CPU. It's almost always the network architecture or the HMI refresh rate.
Roughly speaking, if your cycle time is over 500ms, upgrading the PLC will not solve it. You need to profile the code. I'm not 100% sure, but I bet you have a few 'blocking' functions (like a copious PID loop or a dodgy math block) that are eating your CPU time. Fixing those in software costs exactly $0 in hardware.
I know this might sound anti-progress. I'm not saying never buy a new Siemens PLC. But the default should be 'no.' The default should be: 'Can we fix this with software? Can we buy a spare? Can we train someone?'
After managing these upgrades for nearly a decade, I've learned that the most expensive thing you can do is buy a new PLC for reasons that have nothing to do with actual production output. The lowest quote (in this case, the new PLC) almost always carries a hidden cost: your time, your stress, and your credibility with the maintenance team.
As of January 2025, I still have S7-1200s running perfectly well on lines that are 'officially' two generations old. They don't know they're obsolete. And because we have a spare on the shelf and a guy who knows how to code them, we're saving about $15,000 a year in unnecessary upgrades.