Siemens PLC Choices: Three Scenarios, Three Different Answers

Why I Stopped Giving a Single Answer

I've spent nine years handling Siemens PLC retrofit and upgrade orders. In that time, I've personally made—and documented—14 significant mistakes, totaling roughly $47,000 in wasted budget. The most expensive ones all came from the same root cause: treating every plant as if it needed the same answer.

So I stopped giving one. Here's the split I use now, and the mistakes that forced me to make it.

Three scenarios. Not exhaustive, but they cover most of what I see.

• Scenario A: New line, no legacy baggage.
• Scenario B: Existing S7-300, still running fine.
• Scenario C: Mixed shop, planning a phased transition.

Pick the one that sounds like your plant. Then read the section that applies.

Scenario A: You're Building Something New

If you're starting fresh—new machine, new cell, no existing Siemens hardware to match—this is the easy one. Go S7-1200 for small standalone machines, S7-1500 for anything with real I/O counts or integrated motion.

What I'd avoid: buying used S7-300 hardware to save money. I did this in 2019. Found a "great deal" on a used S7-300 rack for a packaging cell—$3,200 for the full setup, roughly 40% under new S7-1200 pricing. Looked smart on the PO. Then a power module failed in month seven. Lead time on the replacement: six weeks. The line sat idle for most of that.

The $3,200 we "saved" got eaten in the first week of downtime.

Here's the thing: Siemens has been moving S7-300 into its end-of-life phase for a while now. If you want exact stop-sale and support dates, ask your distributor—I don't want to quote a date I'm only half-sure about. But the direction is clear. New projects belong on the current platform, not the outgoing one.

Also: if you're doing anything that touches a corporate network, the S7-1500's integrated security features (aligned with IEC 62443) are worth the premium. I've had to retrofit network isolation on older hardware before. It's not fun, and it's not cheap.

Scenario B: S7-300 Is Running, and You're Wondering If You Should Upgrade

This is where I see the most expensive mistakes. Not because upgrading is wrong—because upgrading too early is.

In 2022, my team inherited a cell running S7-300 hardware that had been stable for eight years. Zero unplanned downtime in that period. Someone in management read an article about Industry 4.0 and decided we needed to "modernize." We planned a full migration to S7-1500 over a scheduled shutdown.

The shutdown was planned for three days. It took six. Integration issues with an old Profibus segment, an HMI that didn't talk to the new CPU the way the datasheet promised, and a contractor who knew TIA Portal but not our process. Total cost: somewhere around $11,000 in lost production plus $4,500 in contractor overtime.

Looking back, I should have pushed back harder. The old system was fine. The business case was "we should be modern"—which isn't a business case.

When upgrade actually makes sense:

• Spare parts are getting hard to source (check your distributor's stock before assuming).
• You have a real cybersecurity requirement—IEC 62443 compliance, customer audits, that kind of thing.
• The old system can't do something the business now needs: data logging, remote access, integration with MES.

If none of those apply, my honest advice is to leave it alone and keep a solid spares inventory. I have mixed feelings about this—part of me likes new hardware, part of me remembers that $15,500 bill—but the numbers don't lie.

Scenario C: You're Running a Mix and Planning a Transition

This is the most common situation and the hardest to give clean advice on. You've got some S7-300, maybe an older S7-200 or Logo!, some S7-1200s from a recent project, and a budget that only covers part of a full migration.

My advice: migrate by bottleneck, not by age.

Find the station that actually costs you money when it goes down—the one with no spares, the one your oldest technician is the only person who can troubleshoot, the one that's holding back a data project. Migrate that one first. Leave the stable, boring stations alone until they give you a reason.

I made this mistake in 2021. We had a mix of S7-300 and S7-200 hardware, and the plan was to replace everything in one fiscal year. Halfway through, the budget got cut. We ended up with a half-migrated line where old and new hardware didn't share a clean network architecture. That compromise cost us more in integration work than doing it sequentially would have.

Two things I'd insist on if I were doing it again:

1. Train your people before the first migration, not after. A TIA Portal-trained team can absorb a phased rollout. An untrained team will fight it at every step.
2. Keep network segments isolated during the transition. Mixing old Profibus and new Profinet without a clear bridge plan is how you get mysterious faults at 2 a.m.

A Quick Word on Market Share (And Why I Won't Quote Numbers)

The search term "Siemens PLC market share by region 2024" comes up a lot, so let me address it directly. I don't have hard data I'd trust enough to publish. I've seen industry reports put Siemens at or near the top globally in PLC shipments, but regional breakdowns vary wildly depending on whether you count by revenue, units, or installed base.

What I can say anecdotally: in the North American shops I've worked with, Siemens presence is strong in process industries and automotive, thinner in discrete manufacturing where you'd see more of the US-based vendors. In Europe, it's closer to default. In Asia, it varies enormously by country.

If you need real numbers, go to the source: Siemens' annual reports, HMS Networks' industrial network market reports, or a paid analyst firm. Don't trust a blog post—including this one—with a percentage that has no methodology behind it.

How to Tell Which Scenario You're In

Three questions. Answer honestly.

1. Is this a new build or an existing line? If new, it's Scenario A. Skip the rest.

2. Can you still source spares for what you're running? Call your distributor before you answer. I've been surprised—in both directions—by what's actually available versus what the lifecycle chart says.

3. Does your team know TIA Portal? If not, factor training into any migration plan. This isn't a box to check later. It's the difference between a three-day shutdown and a six-day one.

If you're still unsure, the conservative answer is usually right: keep what works, stock spares, and migrate when you have a specific reason—not because a brochure told you to.

An informed decision here is worth more than a fast one. I've paid for the fast one. You don't have to.