In late 2018, the procurement manager at our machinery company quit, and I inherited her desk. I knew how to buy sheet metal, fasteners, and bearings. I did not know a PLC from a relay logic diagram. Six-plus years later, I have placed somewhere around $350,000 in Siemens PLC orders. Maybe $320,000 — I would have to pull the ERP report to give you a real number.
Most of the painful lessons came in that first year. I asked three distributors for pricing on “the standard Siemens S7-1500.” One quoted about $900. Another was around $1,400. The third came in above $1,900. Same phrase, same product family, not the same product. A controls engineer finally asked me for the MLFB number, and I thought he was joking.
This article is the checklist I use now. It is not a Siemens PLC programming guide — I do not write PLC code and I do not commission lines. It is a buying checklist for people like me: procurement folks who need to purchase Siemens PLCs without an engineering degree. Seven steps. If you commission S7-1500s daily, you can skip this. If you sign the POs, keep reading.
The headache starts when someone says “we need a Siemens PLC” and nobody writes down what it has to do. Before you contact a vendor, make the engineer or integrator answer these questions:
Some of those words may sound foreign. That is fine. You are not choosing the PLC; you are making sure every supplier quotes the same PLC so you can compare them. A request for quote without an I/O count is not a request for quote. It is a guess.
“We need an S7-1500.” Which one? The S7-1500 family includes CPU 1511, 1513, 1515, 1516, 1517, and 1518, not to mention fail-safe F CPUs and technology CPUs. The list price difference between a 1511 and a 1516 can be two to three times. That is not a small rounding error.
I almost ordered the wrong one once. A customer required functional safety on part of a machine. I looked at the quote for the standard CPU and the F version side by side. The F version was noticeably more expensive, and I asked why we were paying extra for a letter. The engineer explained it patiently: a non-F CPU cannot run a fail-safe program. You can wire the safest I/O modules around it, but if the CPU cannot execute the safety logic, the machine does not meet its safety rating. The extra letter was the entire point.
So make someone name the exact CPU model and give you a reason. If the answer is “that is what we always use,” push a little. I am not saying you will save money every time — often the bigger CPU is justified. But at least know why you are paying for it.
Software does not feel like a procurement problem until it is. When we bought our first S7-1500, I treated the engineering software as a side issue and assumed “software” was somewhere in the budget. That was wrong. TIA Portal is a real line item, and the version your engineer uses matters as much as the CPU part number.
Two checks go on every order now. First, the license: programming an S7-1500 is done in the TIA Portal environment with a STEP 7 package. There are different packages and options, and licenses can be tied to a single workstation or set up as floating licenses. I am not going to quote exact prices here because they move; check current pricing on the Siemens Industry Mall or with your distributor. The question to ask is simple: which STEP 7 license will this project use, and is it already owned?
Second, firmware compatibility. S7-1500 CPUs ship with firmware from the factory. If the firmware version is newer than what your TIA Portal version supports, your project may not open or download cleanly. Ask your supplier directly: “This CPU must run in TIA Portal V17. Will the unit you ship work with that, or will we need to update firmware?” The update is often routine, but it is better to know before the CPU is sitting on your bench.
One more thing: if the person programming the PLC is new to the platform — maybe they have been working through a Siemens PLC programming guide while waiting for the hardware — budget for training. The worst software overruns I have seen did not come from bad PLCs. They came from learning on a live project.
The MLFB is Siemens’ order number. It usually starts with 6ES7, and the full string identifies the product version precisely. Ask your engineer to pull the exact MLFB from the TIA Portal hardware catalog and include it in the request for quote. Do not let a distributor substitute a “similar” number without checking with the engineer first.
When you quote the exact same MLFB, the price spread between suppliers will shrink — but it will not disappear. A difference of 10 to 20 percent is common. Compare the total landed cost: unit price, freight, handling fees, payment terms, and delivery date. The lowest sticker price does not always win once you add those.
This is where the “cheap” quote gets expensive. An S7-1500 CPU is not a complete system. You usually need a 24 VDC power supply, a mounting rail, PROFINET connectors, and industrial Ethernet cables. Signal modules and communication modules are extra. None of those are mystery parts — they just do not always show up on a one-line quote.
One thing that surprised me when we moved from older S7-300 hardware to S7-1500: many S7-1500 CPUs have built-in load memory for ordinary projects, so a SIMATIC Memory Card is not automatically required. If a supplier adds a memory card without explaining why, ask. Sometimes there is a good reason — large program, firmware handling, removable program card. Sometimes it is just an easy add-on.
The easiest way to catch this is to ask for a complete parts list and check every line. If an item has no part number, ask them to label it. If you cannot name what it is, neither should you pay for it.
The cheapest fix is the one made before the cabinet is energized. An S7-1500 runs on 24 VDC. If the supply voltage is out of range, if the polarity is reversed, or if someone has wired a sensor incorrectly, you can damage modules in a way that no warranty covers.
In our shop, opening a live panel is an electrician’s job. I do not open panels, and I do not pretend to be a controls technician. Measuring the DC supply before connecting the PLC, verifying the circuit protection, and checking the ground are all tasks for someone who does this daily. If somebody without industrial experience offers to show you how to measure AC current using a multimeter inside a live control panel, decline the lesson. That kind of work requires proper meter ratings, the right personal protective equipment, and training.
Here is the question I ask before every significant PLC order: when this CPU fails, what is the plan?
In 2024, a CPU on a customer’s line failed on a Tuesday morning. We had no spare. Our regular distributor could deliver in three days. Another supplier could air-freight one for the next morning, with a freight charge I do not remember exactly — a few hundred dollars, I think. Normally I would compare options and get approval, but the customer’s line was down. I signed the air freight. It was the right call, but it was a call we should not have had to make.
Now we keep one spare CPU and one spare power supply for every critical cell. That sounds expensive until a machine sits idle on a Tuesday. The total cost of the spare is usually small compared with one unplanned day of downtime.
It also helps to buy from an active product family. As of early 2025, the S7-1500 line was in active support — check the lifecycle status for your exact MLFB on the Siemens Industry Mall. Active support means replacement modules and firmware updates will be available for years to come. That matters more than a small price difference on day one.
If you only remember three warnings, these are the ones.
Seven checks. Half an hour. It will not replace your controls engineer, and it will not teach you to program the PLC. But it will keep you from ordering “the standard Siemens PLC” and hoping for the best. These days I order a specific CPU, check the software version, and wait for the delivery confirmation like everyone else.