I've been handling Siemens PLC orders for about 6 years now. In my first year (2019), I personally made a mistake that cost my company $3,200 in rework plus a 2-week delay – I ordered an S7-1200 with the wrong firmware version for a TIA Portal project that required the S7-1500's motion control capabilities. That was the day I started maintaining a checklist for my team. Since then, we've caught 47 potential errors using this same checklist.
This checklist is for anyone who's new to Siemens PLCs and needs to avoid the common traps I've fallen into. If you're a maintenance technician, a junior engineer, or a procurement specialist trying to specify the right hardware, this will save you time and money. Here's the thing: it's not just about picking the right model – it's about the ecosystem (TIA Portal version, licensing, compatibility).
This checklist has six steps. Use it every time you start a new PLC project or order hardware.
Most people think they just need a CPU and it'll work. I've learned the hard way that this isn't true. When you order an S7-1200 (e.g., 1214C), you need to check the firmware version against your TIA Portal version. Siemens releases firmware updates frequently, and older TIA Portal versions might not support newer firmware.
In September 2022, I specified an S7-1200 with firmware V4.6 but the project was built in TIA Portal V16 (which only supports up to V4.5). The CPU arrived, and we couldn't upload our program. Cost: $890 in rework plus a week of panic. Now my checklist says: Always match firmware to TIA Portal version – check Siemens Industry Online Support for the compatibility list.
If you're working with an existing machine, read the PLC's actual firmware from the device (via TIA Portal or the display) before ordering replacement parts. Don't assume the manual is correct – things change.
This is where most of the mistakes happen. A PLC isn't just a CPU – it needs the right mix of digital input modules, analog output modules, and sometimes special function modules. But here's what people overlook: the backplane bus current and the 5V supply from the CPU.
I once ordered 8 SM1221 digital input modules for an S7-1200 rack without checking the current budget. The CPU can supply only 1.2A on the bus at 5V, and each SM1221 draws 150mA. 8×150mA = 1.2A exactly – that's maxed out. Adding any future module would cause random failures. Always leave at least 20% headroom on the bus current.
Check the Siemens manual (e.g., S7-1200 System Manual, chapter 3.4) for the current consumption of each module. I recommend using the TIA Portal hardware configurator – it calculates the power budget automatically. Yes, that's obvious, but I've seen engineers skip it because they "know the modules." Don't skip it.
This gets into territory that I'm not a specialist in – motor control and high-power loads – but from a PLC selection perspective, I can tell you this: the difference between a relay and a contactor is not just about size.
A relay is an electrically operated switch with low current ratings (typically 2–10A) and mechanical contacts. A contactor is a heavy-duty relay designed to switch high currents (often tens or hundreds of amps) and is built to withstand arc flashes and frequent switching. Inside a PLC, the digital output modules use relays (or transistors) to control signals. When you need to drive a motor or a heater, you usually connect the PLC's relay output to a contactor's coil – the PLC tells the contactor to switch the big load.
The common mistake? Using a PLC's onboard relay output (e.g., 2A) directly to switch a 5kW motor. The relay contacts weld shut, and the motor never stops. Always use a contactor between the PLC output and any load over 2A (or inductive loads). And remember, contactor coils are inductive – add a freewheeling diode or snubber to protect your PLC output.
It's tempting to think you can just use any relay – but the '[SIMPLE RULE]' advice ignores the nuance between resistive and inductive loads. Contactors have arc chutes and different contact materials. Always check the load type.
I can't stress this enough: TIA Portal licensing is confusing. There are basic, professional, and advanced versions. There are floating licenses, single-user licenses, and trial versions. I learned this in 2020 when I had a customer impatiently waiting for a program, and I couldn't open their project because I only had the basic version and they used the advanced version (which supports all controllers, while basic only supports S7-1200 and S7-1500).
Here's my checklist item: Ask the customer (or your own team) which TIA Portal version and which controllers they use. Then verify that your license covers those controllers. Also, check that the project was created in a compatible version. TIA Portal V15 projects can be opened in V16, but not the other way around. Avoid using the latest version if you're working with existing machines – they might be on V14, and you'll have to convert (which can corrupt certain blocks).
As of my last project in Q1 2025, the current TIA Portal version is V19. Pricing for a professional floating license is about €4,500. This was accurate as of Q4 2024 – the market changes fast, so verify current prices before budgeting.
Many engineers focus only on the ladder logic and forget to document the hardware configuration and parameters. Things like IP addresses, subnet masks, module parameters (e.g., input filter times, output behavior on CPU stop), and the exact part numbers of all modules. Without this, a replacement PLC becomes a nightmare.
I once had to replace an S7-1500 in a rush – the original engineer had left, and the project file was lost. We spent three days reverse-engineering the settings from the running PLC (you can read them back via TIA Portal, but not always all parameters). Now our checklist includes: Export the complete hardware configuration (.amh file) and store it with the project. Also, print a label with the IP address and MAC and attach it to the PLC.
This is another area I'm not an expert in – mechanical enclosure design – but from a field experience perspective, I've seen PLCs fail because they were installed in a metal panel without proper grounding for the communication cables. The cabinet was clean, but the signal cables ran alongside power cables without shielding, causing random communication errors. Separate signal cables from power cables by at least 20cm, and use shielded twisted-pair cables for Profinet. Ground the shield at one end only.
Also, check the ambient temperature. The S7-1200 is rated for 0–60°C, but if it's inside a sealed metal box in direct sunlight, internal temperature can exceed that. I've seen a PLC shut down because of overheating – not a hardware fault, just a design oversight. Always plan for cooling or derating.
In my opinion, the best investment you can make is to spend 30 minutes reading the Siemens Getting Started manual before ordering anything. It's free and saves days of trouble.
This checklist is based on my personal experience and lessons learned since 2019. Things may have evolved – Siemens releases new hardware and software frequently, so always verify against the latest documentation (support.industry.siemens.com). If you need more specific advice, I'd recommend consulting your local Siemens distributor or an experienced system integrator.