I'm an automation engineer who's been handling Siemens PLC selection and commissioning for seven years. I've personally made—and documented—nine significant mistakes, totaling roughly $37,000 in wasted budget. Most of it wasn't dead hardware. It was choosing the wrong PLC for the job in the first place.
We didn't have a formal selection process back then. That changed in 2021, after our third oversized-spec project in a row. Now I maintain our team's selection checklist, and the question I get most from other engineers is a comparison: Siemens PLC S7-300 or a Siemens small PLC like the S7-1200? The hesitation makes sense. The price gap is real, and the capability gap is too. This article is the comparison I wish someone had handed me in 2017.
I'm comparing across five dimensions I've seen cause real problems:
A quick caveat: my experience is based on roughly 60 machine projects and 25 panel retrofits, mostly in food, beverage, and packaging. If you're working in process industries or heavy machinery, your experience might differ.
The S7-300 is a genuine workhorse. It handles complex motion control, larger programs, multiple simultaneous communication protocols, and scales to hundreds of I/O points with remote racks. The small PLC line—S7-1200, plus LOGO! at the low end—is more modest: onboard I/O, a limited number of expansion modules, and a CPU that handles discrete logic and basic analog well but starts sweating under heavy communication loads. (This was back in 2017; the newer S7-1200 revisions have closed the gap, but the class difference is still there.)
Here's the problem: most applications don't need the S7-300's power. In 2017, I specified an S7-300 for a packing line that had maybe 12 inputs and 8 outputs, all discrete. It ran perfectly. It was also embarrassingly oversized. When I worked through the numbers later, an S7-1200 handled the same logic with a faster install time and roughly half the hardware cost. If I remember correctly, the S7-300 config ran about $4,200 in hardware—but don't quote me on that, it was 2017.
The comparison verdict: If your application is mostly discrete logic with fewer than about 20 I/O points, the S7-300's processing power is dead weight. If you need distributed I/O, redundancy, or advanced motion, step up to the S7-300.
In September 2022, I was commissioning a small conveyor system. I selected the S7-1200 with a relay output module. The motor was 1.5 kW. In my head, the relay output on the PLC would switch the motor directly. It worked for about three cycles. Then the relay contacts welded closed, and the conveyor ran until a safety limit switch cut main power.
That incident taught me the difference between a relay and a contactor better than any textbook. A relay is a control-circuit device. It switches low-power loads: indicator lamps, small solenoid valves, contactor coils. A contactor is a power-circuit device, built with heavier contacts and arc suppression to handle motor loads and large inductive loads. Contactors are rated by IEC utilization categories—AC-3 for motors, AC-1 for resistive loads. A PLC relay output physically cannot handle a motor's starting current. The contacts are too small, and there's no arc quenching to speak of.
This matters when comparing Siemens PLC families because both the S7-300 and S7-1200 offer relay output modules, and both share the same fundamental limitation. The correct architecture is always the same:
PLC relay output → drives contactor coil → contactor switches the motor
The PLC lives in the control circuit; the contactor lives in the power circuit. Mix them once, and you'll spend a weekend replacing welded contacts.
The difference between a relay and a contactor isn't just a quiz question. It's a panel design decision. Choose based on the output module terminal capacity and current ratings—not on any expectation that one PLC family will save you from contactors. It won't.
The comparison verdict: On this dimension, the S7-300 and S7-1200 are more alike than different. Both need external contactors for motor loads. (Should mention: this mistake happens with dedicated controllers too. I've seen Honeywell HVAC controllers with relay outputs misused to switch fan motors directly. Same result, same welded contacts.)
Both families program in TIA Portal, which is a relief compared to the old days. (Should mention: if you're inheriting an S7-300 project from the Step 7 Classic era, that's a different beast. I've had to convert a few, and it's rarely painless.)
A small PLC is faster to get right. Basic ladder logic, onboard I/O, done. LOGO! has its own software and is nearly approachable enough for someone with zero programming background. For a small integrator or a plant maintenance team, that ease of troubleshooting matters every single day.
The S7-300 rewards experienced programmers. It gives you organizational blocks, system function blocks, distributed I/O via PROFIBUS or PROFINET, and enough rope to hang yourself if you don't know what you're doing. I've inherited S7-300 code that looked like an eldritch tower of jumps and indirect addressing. It worked. Nobody wanted to touch it. More often than not, that's the maintenance reality of a long-serving S7-300.
The comparison verdict: Match the platform to the team. A generalist technician can troubleshoot an S7-1200 with modest training. An undocumented S7-300 with complex logic is a maintenance trap, and the cost of that trap rarely appears in the initial hardware quote.
Here's the comparison dimension where I learned that "the best PLC" isn't always the right answer.
In early 2023, I proposed retrofitting a rooftop unit's controls with an S7-1200. The facility's building management system was Honeywell-based, and the existing infrastructure used Honeywell controllers throughout. My proposal looked solid—the S7-1200 could handle all the sequence-of-operation logic, and TIA Portal gave us flexible scheduling.
Had two days to put the proposal together. Normally I'd have done a site integration audit first, but there was no time. So I went with the S7-1200 based on specs alone.
What I underestimated was integration. To talk BACnet to the existing BMS, I needed gateways, custom mapping, and a lot of time on the phone with support engineers. The project ran over timeline and over budget. A Honeywell HVAC control panel would have dropped into the existing BMS with native BACnet communication and pre-built sequence-of-operation templates for RTU control.
To be fair, the S7-1200 wasn't the wrong controller in isolation. It was the wrong ecosystem for that facility. And this isn't a knock on Siemens—it's a lesson about matching the controller to the installed base.
The comparison verdict: For commercial building automation where the site already runs a Honeywell BMS, a dedicated Honeywell HVAC control panel is usually the lowest-integration-cost path. A general-purpose Siemens PLC is a great tool, but it's not always the best tool for the job. In hindsight, I should have pushed for the integration audit before promising a timeline. The facility manager was pushing, and I made the call with incomplete information.
This dimension never shows up on a datasheet, and it matters more than most people expect.
When a PLC fails on a production line, every hour of downtime is money. The fix is usually a replacement module. How fast you can get it depends on logistics, not specs.
An S7-1200 CPU is small—roughly the footprint of a hardcover novel. It fits in a UPS flat rate box or a USPS Priority Mail flat rate box. We keep one spare S7-1200 in a UPS flat rate box, ready to ship at a fixed cost regardless of weight (up to 70 lbs). That's a significant advantage when customers run 24/7.
S7-300 modules are bulkier. If you're shipping a rack plus power supply plus I/O, you're paying dimensional weight, and the flat rate options don't fit. According to USPS (usps.com), flat rate boxes ship at a fixed price regardless of weight—which is exactly why small PLCs win on this dimension. The part cost is lower too. A spare S7-1200 is a fraction of an equivalent S7-300 configuration, so carrying spares on the shelf is justifiable for small integrators.
The comparison verdict: For field service and distributed machinery, the small PLC line wins on shipping, sparing, and replaceability. The S7-300 requires more planning and more money to keep spares in the field.
I don't have a universal answer—if someone gives you one without asking about your application, they're selling something. Here's my scenario-based shortcut:
Honestly, I'm not sure why the S7-300 remains the default spec in so many machine builder templates. My best guess is legacy code and habit. But habit costs money. Switching to right-sized controllers on our last three projects cut hardware costs by an average of 22%, and commissioning time dropped noticeably.
If I could redo that first packing line job, I'd pick the S7-1200 and use the saved budget for proper documentation. But given what I knew then—which was close to nothing about right-sizing—the mistake was inevitable. The good news: I've only made it once.
The trend toward more efficient, right-sized control design is real, and the digital tools now available make it easier to validate your choice before you commit. I still respect the S7-300. I also think it gets specified more often than it's genuinely needed. (As of 2025, at least. Things change.)