Let me start with a basic repair analogy, because it has saved me from a lot of bad procurement decisions.
A search for 'fuel filter Chevy Silverado 1500' returns a part number, a spring clamp, and a plan that takes about half an hour. A search for 'how to replace fuel pump' is different. The pump sits inside the tank or under a bed access plate; you need to relieve pressure, watch your lines, and plan for a few stubborn bolts. Same fuel system. Different risk level.
I keep that contrast in mind when I review control system specs. I am the quality/compliance reviewer for an industrial automation supplier. I review about 200 purchase orders and acceptance documents per year. In the first half of 2024, I rejected 11 percent of first-time deliveries for missing documentation and configuration mismatches. The pattern was not broken Siemens PLC parts. It was people treating a PLC like an interchangeable part, when it is closer to the fuel pump than to the fuel filter.
The surface problem usually starts with a comparison: Siemens PLC vs Allen-Bradley. Or it starts with a search that reduces an entire platform decision to one scoreboard number.
I have seen that phrase in too many procurement notes to ignore it. Market share sounds factual. It appears in market reports and feels like a safe choice. But a market share number is a historical aggregate. It does not tell you why those buyers chose one platform, what machines it currently controls, or whether your local supply chain can support the same decision.
In Mexico, the local part matters even more. A global chart cannot tell you whether a distributor in your industrial region keeps a Siemens S7-1200 CPU in stock, or whether the local support engineer has experience with your remote I/O network. It also cannot tell you if your own plant is full of electricians who already know Allen-Bradley PLCs. Your installed base matters more than someone else's installed base.
The better question is not who has the largest installed base. It is: which system can my team support for the next ten years?
The deeper reason so many comparisons go wrong is that a PLC is a system anchor, not a single component. If someone searches for 'plc logo siemens,' they may be looking for the Siemens LOGO! controller, not for the Siemens company logo. LOGO! is a real product family, and it is the right answer for some small machines. But comparing LOGO! with an S7-1500 would be a waste of time until you define the job. Same brand, different class, different support requirements.
A 510 thread battery charger makes the same point. A charger with the right thread will physically connect to the battery, but the charging profile can still be wrong. A matching connector is not a matching system. PLC comparison works that way too. Two controllers may both have Ethernet ports, but the program development environment, the data handling, the HMI integration, and the cyber security support are not automatically the same.
When I see a quote that substitutes a different PLC for a specified Siemens PLC, I do not start with price. I start by asking what the lower-priced option removed from the overall project. During a Q1 2024 audit, I flagged a quote that appeared to reduce first cost by 12 percent. By the time I added the cost of a second programming package, a field service visit, and a separate spare parts inventory, the 12 percent saving had turned into an estimated 19 percent extra cost over the first year. No one planned that. It happened because the comparison was based on CPU model numbers, not on system delivery.
On paper, two PLCs may look identical. In operation, they are different because the people, software and spare parts around them are different.
The expensive failure is rarely an electrical failure. It is a production line stopped because no one can log in to the controller. It is an OEM that pays for a second software environment, or a maintenance team that has to support three PLC brands with no one who knows all three.
I have seen the same $2,400 'saving' appear in multiple projects. The hidden cost showed up in different places: once in software licenses, once in commissioning delays, once in overnight freight. The saving was real at the component level and imaginary at the system level.
Training is the first cost people miss. A Siemens PLC in the S7-1200 or S7-1500 family is programmed through TIA Portal. A different PLC uses a different environment. If your engineers already work in TIA Portal, starting a project on another platform adds cost on day one. That is not a brand opinion. It is labor economics.
Security compliance adds another layer. I write IEC 62443 or plant-specific OT security requirements into the same line as the CPU model. A processor may look good in a datasheet but still not fit the approved security architecture. Ignoring that exposes the whole system to a risk that is not visible in a comparison table.
If you are evaluating a Siemens PLC, start with the system requirements, not the market chart. Here is the short version I use in reviews.
Then, and only then, compare quotes. If an Allen-Bradley PLC fits the same system-level requirements and your team can support it, choose it. If a Siemens PLC fits better, choose that. The selection should come from installed capability and lifecycle cost, not from a number that describes what other people bought last year.
The cheapest PLC quote is not the most expensive mistake. Choosing a popular platform without a support plan is. High quality is what happens when someone writes the specification tightly enough to keep the next person honest. That is the part worth paying for.