Siemens PLCs: 7 Questions I Wish I'd Asked Before My First S7-1200 Build

Why this FAQ exists (short version)

I've been specifying and commissioning Siemens PLCs since 2017. In that time I've made roughly 14 mistakes big enough to document—about $41,000 in wasted budget, rework, and downtime. This is the list of questions I now ask myself (and my team) before every build. If you're buying your first S7-1200 or training a new engineer, start here.

1. Which S7-1200 specifications actually matter in the field?

The datasheet has 40+ numbers. About six of them decide whether your project works.

• Work memory — CPU 1214C has 100 KB, 1215C has 125 KB, 1217C has 150 KB. That sounds generous until you add Modbus libraries and HMI tags. I ran out of memory on a 1214C in 2019 and had to upgrade mid-project.
• Power supply type — DC/DC/DC, AC/DC/RLY, and AC/DC/RLY variants are not interchangeable. I ordered the wrong one in my first year and it cost us $1,900 in rework plus two days.
• Onboard I/O count — 1211C is 6 DI/4 DO, 1214C is 14 DI/10 DO, 1215C adds 2 AI/2 AO.
• PROFINET ports — one on the 1211C, two on 1212C and up. If you need a ring topology or a separate HMI network, that second port is not optional.
• Ambient temperature — 0 to 55 °C horizontal mounting, 0 to 50 °C vertical. Cabinets in uninsulated plant rooms hit 55 °C more often than you'd think.
• Instruction execution — 0.08 µs per boolean instruction on the 1214C and up. Honestly, this is almost never your bottleneck. I've seen one machine where scan time mattered, and it was doing motion control.

Should mention: verify current figures against the official Siemens S7-1200 system manual before you order. Datasheets get revised, and I've been burned by a spec sheet that was two revisions old.

2. Is Siemens PLC programming basics harder than people say?

The first week in TIA Portal is genuinely rough. The second week is fine. The third week you wonder why anyone complains.

The software isn't the hard part. The concepts are: scan cycle behavior, the difference between FCs and FBs, why your DB keeps going to zero, and data type mismatches (feeding an Int into a Real input and wondering why the math is wrong—guilty, 2018). Siemens programs to IEC 61131-3, so Ladder (LAD), Function Block Diagram (FBD), and Structured Control Language (SCL) all work. Start in LAD. Move to SCL when you hit array handling or string parsing.

My advice: buy a cheap 1211C and a copy of TIA Portal, write a traffic light, then a pump alternation, then a recipe handling block. Three exercises, maybe 20 hours total. That's a more honest entry point than any 40-hour course.

3. Doesn't the cheapest option save money?

It did not save me money. Not once. Not in eight years.

In September 2022 I bought a third-party "compatible" analog input module for a packaging line—$340 versus $890 for the original. It worked for 11 days. Then it dropped its channel mid-shift, the line faulted out, and we spent two days diagnosing and swapping it during a production window. Downtime cost: about $8,400. Add the emergency freight and the customer's trust, which cost more than the money.

I still kick myself for that one. If I'd paid the extra $550, I'd have had a warranty and a phone number to call.

To be fair, cheapest isn't always wrong. I've used refurbished original modules successfully when I tested them properly (see the next question). But the probability is not on your side, and the failure mode is not graceful—it's usually an intermittent fault during peak production.

4. Can I buy used or third-party Siemens PLC hardware safely?

Used original hardware: yes, with a checklist. Third-party clones: rarely, and never on a line you can't afford to stop.

For used modules, I verify four things before payment: the exact order number (6ES7... string), firmware version readable in TIA Portal, the seller's return window, and where it lived. A module pulled from a climate-controlled cabinet is a different animal from one that sat in a wet basement for five years.

I also got burned by a non-original programming cable in 2021—worked fine until a TIA Portal update, then threw connection errors for half a day. That one only cost me four hours, but four hours on a commissioning deadline is real money. That's when I made the rule: original cable, always. It's a $200 item that stays in the bag for a decade.

5. Inverter, generator, or battery—what should back up the PLC?

Here's the difference people mix up: an inverter converts DC (usually from a battery bank) into clean AC, while a generator produces AC directly from fuel and a spinning alternator. The inverter gives you a stable waveform and instant transfer. The generator gives you long runtime and high capacity, but dirty power and a start-up gap.

For the PLC and control circuit specifically, I use a battery plus inverter setup (essentially a UPS) with a proper emergency battery charger keeping the bank topped off. Transfer is in milliseconds, the waveform is pure sine, and the PLC never sees a dip.

I didn't fully appreciate this until March 2023, when a generator transfer took 1.8 seconds during a site test and my PLC rebooted mid-cycle. Nothing was damaged, but we lost the recipe state and the operator had to re-run the batch. Since then, no PLC of mine sits directly on a generator without a UPS in front of it. Important: if you're sizing a UPS inverter, insist on pure sine wave output. Modified sine wave and switching power supplies are a bad combination.

6. Will a third-party control panel talk to a Siemens PLC?

Sometimes. PROFINET, Modbus TCP through a gateway, or OPC UA with the right license—those paths work. Proprietary serial protocols usually don't, and the workaround costs more than the panel.

I once spent a week bridging a third-party operator panel to an S7-1200 through a protocol converter. Latency was tolerable, but every firmware update broke the converter's configuration file. We ended up replacing the panel.

Worth clarifying, because I get asked odd versions of this: a Truma control panel on a camper and a Siemens HMI are not in the same category—different bus, different tolerances, different safety requirements. If a supplier compares them to justify a swap, walk away. For any third-party panel, get the protocol and certification in writing before you buy.

7. What's on your pre-build checklist now?

Seven lines. We've caught 47 potential errors with it in the past 18 months, and I keep a printed copy in my bag.

1. Order number and firmware version confirmed against the system manual
2. Power supply type matched to the field wiring plan
3. I/O count plus 20% spare capacity
4. PROFINET port count and topology
5. Cabinet ambient temperature measured, not estimated
6. Power source: UPS/inverter or generator, decided before installation
7. Spare module on the shelf for every CPU model we deploy

That last one isn't about Siemens. That's about the 3 a.m. phone call I'd rather not take again.