If you've been told to "order a Siemens PLC" and you're not entirely sure what that involves, this is for you. I'm an office administrator for a mid-sized industrial controls company—about 60 employees, two locations, 12 vendors under my management. I handle roughly $300,000 a year in automation-related purchases. This isn't a programming tutorial; it's the procurement checklist I've built over five years of buying Siemens PLCs. Most PLC buying advice comes from engineers who've never had to manage a lead-time surprise. If you're designing a plant-wide control system, this is too basic for you. But if you're buying PLCs for smaller machines, replacements, or panel builds, here are the six steps I run through before every order.
Before searching for a model number, get the requirements in writing. I ask five questions on every project:
I learned this one the hard way. I ordered an S7-1200 for a pumping job without double-checking the analog input count. I assumed the engineer had already verified the I/O list. The base CPU has only 2 analog inputs; the application needed 8. We added an SM 1231 analog module and paid for expedited shipping—about $600 and a week of schedule lost to a question I never asked.
If the PLC is going into a sprinkler system control panel, this step matters even more. Life-safety installations carry compliance requirements, and the parts an inspector sees—the PLC, power supply, battery charger—must match the approved design. You don't substitute something "similar" on a fire-protection system.
The SIMATIC S7 isn't one product. When someone says "Siemens PLC," they usually mean one of three lines.
The S7-1200 is the workhorse for small machines, conveyors, and straightforward panel builds. It's compact and reasonably priced, which is why it's my default recommendation for single-machine applications.
The S7-1500 targets complex automation—high-speed lines, multi-axis motion, serious diagnostics, and integrated security. It costs considerably more, but for the right project, that gap is justified.
The S7-300 is the legacy generation. Plenty of plants in Mexico still run on S7-300 hardware, and I still get replacement module orders from those customers. But Siemens has announced end-of-life for various S7-300 modules. If you depend on it, check the lifecycle status page and stock spares while they're still available.
On the endless advantages/disadvantages question:
Advantages: TIA Portal is a genuinely unified engineering environment—PLC, HMI, and drives in one place. That saves commissioning time. Siemens' market position in Mexico also means local training, distributors, and support are realistically available. And newer S7 models align with IEC 61131-3 and IEC 62443, making compliance conversations easier.
Disadvantages: Licensing costs catch people off guard. TIA Portal licenses can exceed the CPU price once you add the packages you need. The ecosystem also creates lock-in—after your team builds templates and library blocks, switching becomes painful. And code doesn't always transfer cleanly between S7-1200 and S7-1500; you can end up maintaining two versions of your standard logic.
To be fair, those downsides aren't unique to Siemens. They're just real. I went back and forth on a 1200 vs. 1500 decision for a client recently and ended up choosing the 1500. On paper the 1200 met the current I/O count. But the client hinted at a second line, and the 1500's headroom avoided a future migration. It's not glamorous, but procurement decisions usually come down to trade-offs like this.
A PLC without the right programming software is a paperweight. Before ordering, confirm:
I've seen this go wrong when a new S7-1200 ships with firmware requiring a newer TIA Portal than the technician has installed. Firmware upgrades exist, but they're not right-click-and-go. We lost two days once. Now I ask: "What firmware is on the CPU?" and "What TIA Portal version is already installed?" The vendor won't always volunteer that information.
Every PLC I order ends up in an enclosure with a dozen other components. For a recent commercial building's sprinkler system control panel, the purchase list included:
The charger and battery backup aren't side quests—they're part of the approval package on life-safety installations. And a practical note: before powering up any new panel, verify continuity. If you're not sure which multimeter setting that is, look for the symbol like a series of gradually bigger arcs, roughly a sideways sound wave: ")))." Touch the probes together, get the beep, and you know the meter is working. Then test your wiring. It's a five-minute check that prevents an afternoon of troubleshooting.
Siemens' PLC market share in Mexico is substantial. Most market analyses put Siemens among the top industrial automation brands here, competing with Rockwell, Mitsubishi, and other global players. In practical procurement terms, that means good local availability of Siemens hardware compared to niche brands. We routinely get S7-1200 CPUs from local distributor stock within a few days.
But "market share" doesn't mean every module is on a shelf. Certain CPU variants and expansion modules are built to order, with lead times of four to eight weeks. I now call two or three distributors and ask directly: "Is it physically in your local warehouse, or does it ship from abroad?" "It's in the system" isn't the same as "it's in the country." For deadline-driven projects, I'll pay a small premium for the part that's actually on a shelf.
The CPU price is a small fraction of the total cost, a fact I wish more first-time buyers understood. My worksheet includes:
The first time I itemized a full S7-1200 setup, the software license cost more than the CPU. The signal modules cost more than the PLC itself. Nobody likes that surprise, but it's better to see it on a quote than at a budget review.
On cost, let me also say this about small orders. I'm often buying one or two CPUs and a handful of modules, which makes me the "small" customer. I can tell when a distributor is barely tolerating my order, and those vendors lost my trust. The ones who treated a $1,500 order with the same professionalism as a $150,000 order are the ones I keep going back to. Today's small order can become next year's big contract.
Assuming "same specifications" means identical products. It doesn't. "Supports PROFINET" can mean onboard or via a separate communications processor. Verify the datasheet, and get the seller to confirm compatibility in writing.
Not pinning down delivery dates. I once told a supplier "as soon as possible," and they heard "whenever it's ready." The quote said one week; the shipment took a month because the inventory was overseas. Now I put a calendar date on every order, not a phrase.
Skipping firmware compatibility checks. It happened to me, and the forums are full of the same story. Check the TIA Portal/firmware compatibility matrix before ordering, not after your technician stares at a mismatch on screen.
Treating supporting components as afterthoughts. On a sprinkler system control panel, the battery and its charger are part of the safety function. We've used the QUIQ HF/PFC battery charger on these panels without problems, but the point is to pick certified components during design, not as a last-minute workaround.
No such thing as a perfect PLC purchase exists. The goal is to remove surprises before they reach your desk. Itemized requirements, honest compatibility checks, and realistic lead times get you most of the way. The last piece is who you choose to buy from.