I've handled purchasing for a mid-sized manufacturing plant since 2020—60-plus orders a year, most of it Siemens PLC gear. S7-1200s, TIA Portal licenses, field wiring, the works. Along the way, I've collected the same questions over and over, from our engineers, from maintenance staff, and from you. Here are the answers, updated May 2025.
If you're new, the model numbers aren't in order of importance. They're separate product lines that share the "S7" prefix.
The S7-300 is legacy. Siemens has been steering new projects away from it for years. Replacement modules are still available, but you don't want to build something new around hardware that's slowly being phased out. It gets harder to find parts, and the migration path to newer systems is no fun.
The short version:
In 2024, our controls engineer specified an S7-1500 for a job that needed about 40 I/O. It worked perfectly and cost about three times what an S7-1200 would have. Don't over-spec just because a vendor says "premium."
Almost always the S7-1200. Specifically the CPU 1214C or 1215C DC/DC/DC. It's the right size for most small automation work—well, the 1214C is. The 1211C saves money but leaves almost no room to grow.
Don't buy the cheapest CPU to keep the initial cost low. The price gap between the 1211C and 1214C is way smaller than the cost of finding out mid-project that you've run out of I/O. Spare I/O is cheap at purchase time and expensive in the field.
And check your power supply. PLCs run on 24V DC. A machine once sat on our floor for three weeks waiting on a 24V power supply that someone forgot to order. The PLC was fine. The schedule wasn't.
I get this one a lot, mostly from schools and training centers. In Indonesia, a complete trainer set—S7-1200 CPU, digital I/O module, push buttons, LED simulators, 24V power supply, and a training board—typically lands between IDR 18 million and IDR 35 million. That's as of May 2025, and it varies with the CPU choice and whether you add an HMI or a TIA Portal license.
Cheaper packages often use the smaller 1211C and skip the software license. If you're teaching, the license matters more than extra LEDs. Students need to write, download, and debug real programs, not just watch lights blink.
Before you order, ask for an itemized quote: CPU model, I/O modules, software license (yes/no), warranty. If a vendor can't provide that now, they probably won't provide a proper invoice later. I learned that in 2022 when a vendor's handwritten receipt got our expense report rejected, and I covered the cost out of the department budget.
You'd think that's a smart budget move. Sometimes it is. Usually, it's a trap.
Used PLCs come with two big risks. First, old firmware that's a pain to connect to current TIA Portal versions. Second—and this is the one people don't think about—used PLCs often carry leftover passwords, licenses, and project data from the previous owner. You can't always wipe them the way you'd expect, and a locked PLC is basically a brick.
I have a used S7-1200 in my drawer that we bought for IDR 5 million. Two years later it's still there, because nobody can unlock it. Buy used only from a seller who demonstrates it working and includes the original serial number. Even then, expect no warranty and no support.
Yes. There's no way around it. If you want to program an S7-1200 or S7-1500, you need STEP 7 in the TIA Portal environment. TIA Portal supports all five IEC 61131-3 languages, though in practice nearly everyone uses Ladder or FBD.
The most frustrating part of TIA Portal is the versioning. You'd think any version could open any project, but that's not true. Newer versions can open older projects; older versions can't open newer ones. At our site, we standardized on one version and made everyone stick with it. Otherwise you'll spend a weekend chasing compatibility errors that make no sense.
If you're learning, Siemens SITRAIN has courses, and there's a free 21-day trial of TIA Portal. Learn the software before buying hardware—that's the cheapest path by far.
This one comes from the maintenance side, not the PLC side. But I've had forklift and generator questions cross my desk enough times that I know the answer by heart.
Not always. A fuel pump that's failing but still kinda working can keep the engine running well enough that the ECU never complains. When fuel pressure drops too far, you might get a code like P0087 (fuel rail pressure too low) or a lean condition code (P0171 or P0174). But if the pump dies completely, you get a crank-no-start with no code at all on a lot of vehicles.
"A code is a clue, not a verdict."
That's what our senior tech repeats, and he's right. Check actual fuel pressure with a gauge before you replace anything.
Classic situation, and it's easy to guess wrong. A forklift that won't crank often gets a new starter before the real problem is found. Before you buy one, check in this order:
Our maintenance crew replaced a starter on one of our 7-series units and still had no crank. The seat switch wire had rubbed through underneath. A $3 repair after a $400 mistake.
You'll need this more than you expect—especially when a machine stops randomly and the engineer swears the sensor cable is bad.
First, turn off power. Always. You're measuring resistance, not voltage. Then set your multimeter to ohms (the Ω symbol). If it's not auto-ranging, start at the lowest range. Touch the probes together and confirm you read near 0Ω—that's your lead resistance. Finally, touch one probe to each end of the wire.
Near 0Ω means the wire is continuous. "OL" means open circuit—the wire is broken somewhere. Anything in between is suspicious: corrosion, a bad connection, or internal damage.
Here's the trick I picked up from our electrical tech: wiggle the wire while measuring. If the reading jumps around, it's an intermittent break inside the insulation. Replace the wire. Don't patch it.