If you've ever opened a control panel and felt a little lost, you're not alone. I'm a quality compliance manager at an industrial automation company. I review every control panel before it goes out the door—roughly 200+ items a year—and in 2024, I rejected about 9% of first deliveries. Usually not for big engineering failures. Usually for small spec violations that should've been caught earlier.
Before we talk about Siemens PLCs and the 2025 market share battle, let's answer the basics: what is a control panel?
A control panel is an enclosure (think of it as the nervous system cabinet) that houses the electrical components needed to control a machine or process. It's where power comes in, signals get processed, and outputs go out to motors, valves, heaters, or generators.
Typical contents include:
The PLC is the brain. On a Siemens-focused panel, that's often a Siemens S7-1200 for small machines or a S7-1500 for larger automation. The rest of the panel is the body: it feeds power, switches loads, and protects the brain from dirty electrical noise.
I get asked about PLC market share 2025: Siemens vs. Rockwell a lot. I don't have hard data on the latest ARC report in front of me—that's not the kind of thing I maintain in-house. Based on the projects I see and the vendors on approved lists, my sense is that Siemens and Rockwell are still trading the top two spots globally, with Siemens stronger in Europe and Asia and Rockwell stronger in North America.
For small PLCs, Siemens has a particular advantage in installed base and training. If you've worked with TIA Portal, you know it's not perfect—but it does two things well: it keeps all devices in one project, and it gives you a common data structure. For someone maintaining panels, that consistency is worth more than any benchmark.
Let's say you're building a diesel whole home generator package. The control panel has to detect grid loss, start the diesel engine, confirm frequency and voltage, then switch the house load over to the generator. When grid power returns, it has to switch back and do a cooldown run. That sequence is safety-critical.
In this scenario, I'd push for a real PLC, not just relays. A small Siemens PLC like the S7-1200 gives you a defined sequence, a fault log, and a way to adjust timers without rewiring. I've rejected panels where the entire grid-fail logic was done with a single relay and no time delay. It works in the shop; it fails in a storm.
The most frustrating part of this job is seeing the same simple issue repeatedly. You'd think a written sequence description would prevent interpretation differences, but it doesn't. That's why I always require a signed validation test procedure for generator controls. Five minutes of verification beats five days of correction.
Now consider a completely different situation. Suppose you're automating a small packaging station that takes spark plugs, inserts them into a printed spark plug box, and closes the flaps. You need a few sensors, two pneumatic cylinders, and a conveyor motor. Do you need a full TIA Portal setup with an HMI and remote maintenance?
Probably not. This is where a Siemens small PLC shines. The S7-1200 with 10–20 I/O points, or even a LOGO! 8 for simpler sequences, is easier to wire, easier to program, and easier to replace when a maintenance tech is standing around waiting for a spare. In this scenario, "bigger is better" is the wrong instinct. Buy the smallest PLC that covers your worst-case I/O count, and leave room for two spare inputs and one spare output.
This is one of those places where my advice usually surprises customers. They expect a quality inspector to demand overbuilt everything. But overbuilding a spark plug box line with a 100-point PLC just makes the panel more expensive and the logic harder to read.
The third scenario is the relay panel that's been running for twenty years. It still works, but troubleshooting is a nightmare. Every time a sequence stops, someone has to chase wires with a multimeter and try to remember what relay K4 actually does.
In this case, I'd call a PLC retrofit a preventive maintenance decision. A small Siemens PLC with a labeled I/O list and a simple program is the cheapest insurance you can buy. You'll likely find the old panel is missing two things: proper short-circuit protection and an emergency stop circuit that meets current standards. According to NFPA 79 (the US standard for industrial machinery) and IEC 60204-1 (the international equivalent), an e-stop circuit can't rely on a PLC alone. I've seen compliance failures here more than anywhere else.
Granted, a retrofit requires work upfront: drawing the existing wiring, mapping inputs and outputs, and testing edge cases. But I'd argue it's worth it. Prevention is almost always cheaper than an unplanned midnight callout.
If you're not sure, ask three questions:
The honest answer is that "what is a control panel" isn't about the box on the wall. It's about a risk decision: how much control logic you need, how much you're willing to spend, and what happens when it fails.
Before you approve any panel:
I'll be the first to admit that no one gets this perfect on the first try. I wish I had tracked every mistake I've caught over the past four years—I'd have a better statistic than the 9% rejection rate. What I can say from experience is that the panels that pass are not the ones with the fanciest PLCs; they're the ones built by people who assumed something could go wrong.