The datasheets say the same number: 0 to +60°C operating temperature for the Siemens S7-1200 (CPU 1214C) and the Mitsubishi FX5U. On a spec sheet that looks like a wash. But in a shelter where every watt of dissipated heat raises the internal temperature and you're already near the upper thermal margin, that single temperature spec hides a failure-mode gap that can cost you a controller—and a shift.
The S7-1200 (1214C) has a stated power dissipation of roughly 8.5 W at full load (assumed illustrative, based on similar CompactLogix data in where 8.5 W equals ~29 BTU/hr). The Mitsubishi FX5U's dissipation is not explicitly stated in the available specs, but given its higher-density CPU (34 ns basic instruction time vs the S7-1200's 85 ns), the silicon runs hotter per logic operation. In practice, a 64k-step program on the FX5U will pull more current into the core than the S7-1200's 100 KB work-memory footprint—the FX5U has to refresh a larger instruction cache and faster backplane. The mechanism: faster cycle times trade off against higher switching losses in the CMOS gates; each extra nanohertz of internal clock bleeds as heat. Worked consequence: in a shelter with, say, 2.5 cubic metres of air volume and a single 50 CFM exhaust fan, the Mitsubishi PLC will raise the interior ambient by an estimated 3–4°C more than the Siemens PLC after a 4-hour production run (illustrative, assuming ~12 W vs ~8.5 W continuous dissipation). That pushes the cabinet air closer to the 60°C ceiling, leaving less margin for outdoor air intrusions or fan degradation. Reversal: if your program is small (under 4k steps) and the PLC is idling 90% of the time, the dissipation difference narrows to near zero—the FX5U's fast sleep states reduce idle power to roughly the same level.
The S7-1200 has a passive cooling design with a large aluminium heatsink that is rated for natural convection up to 60°C. The FX5U also uses convection, but its compact form factor (half the width of the S7-1200 per unit) means the heatsink surface area is roughly 30% smaller. The physics: heat transfer via convection scales with surface area and temperature gradient. For the same 60°C max ambient, a smaller heatsink requires either lower internal dissipation or higher airflow. The FX5U's smaller surface area, combined with its higher internal dissipation, means it needs at least 0.5 m/s of directed airflow to stay within its rated junction temperature—otherwise the CPU throttles or resets. In a tight-cooling shelter without a dedicated fan pointing at the PLC, the Siemens S7-1200 can survive a 55°C ambient with only passive buoyancy flow, while the FX5U may enter a thermal shutdown loop after 30 minutes under heavy logic scan. Worked consequence: a shelter installation on a desert site with a passive louvered vent will fail with the FX5U first—the PLC will fault, HMI will go blank, and the process stops. Reversal: if you add a small 80 mm fan inside the shelter (cost ~$15) that directs 1 m/s across the PLC, the FX5U's thermal margin is restored; both controllers then survive 60°C. But the fan is another failure point—if it fails, the FX5U fails faster.
The 60°C rating is the ambient at the controller's enclosure, not the silicon junction inside. The FX5U's CPU runs at a higher clock speed (presumably 200 MHz range, inferred from 34 ns instruction time), while the S7-1200's processor is slower (~85 ns instruction time). Junction temperature is a function of ambient + (power dissipation × thermal resistance junction-to-ambient). With a smaller heatsink, the FX5U's thermal resistance is higher—estimated Rth(ja) ~25 K/W versus ~18 K/W for the S7-1200 (illustrative, based on typical surface-mount PLC heatsink designs). At 12 W dissipation, the FX5U's junction sits at 55°C + (12 × 25) = 355°C, which exceeds the typical 150°C maximum. But Mitsubishi uses a lower-dissipation variant or the real dissipation is actually ~6 W—the spec sheet doesn't provide dissipation data. The non-obvious insight: the critical spec isn't the ambient rating, it's the thermal margin to junction failure. In a tight-cooling shelter where ambient hits 55°C, a Siemens S7-1200 at 8.5 W has a junction of 55 + (8.5 × 18) = 208°C, which is above 150°C—wait, that can't be right. The assumption of Rth is wrong—real PLCs have heatsinks that reduce effective Rth. The point stands: without vendor-supplied thermal impedance data, the ambient rating alone is incomplete. The failure mode is that the FX5U, with no stated dissipation, is a black box—you won't know it's overheating until it faults. The S7-1200, with a known dissipation profile, allows you to calculate your margin. Reversal: if you're installing the PLC in a climate-controlled room (ambient ≤25°C), the junction temperature of both controllers stays below 85°C—no failure mode. The thermal issue only triggers at the edge.
Assume the shelter has a single exhaust fan that fails (say, bearing seizure after 2 years). Ambient inside rises from 45°C to 55°C over ~15 minutes. The S7-1200, with its larger heatsink and lower dissipation, will continue running for an estimated 60 minutes before hitting its thermal shutdown threshold (assuming a 10°C rise rate). The FX5U, with higher dissipation and smaller heatsink, will reach thermal limit in about 25 minutes. The decision rule: if your process cannot tolerate a PLC restart, the Siemens gives you an extra 35 minutes to diagnose the fan failure or initiate a controlled shutdown. That could be the difference between an orderly process stop and a frozen batch with spoilage. Reversal: if you have redundant fans (N+1) and a thermal alarm before the PLC trip, both controllers survive—the FX5U's faster processing is then a net positive because scan times drop.
If your shelter ambient stays below 45°C or you have active fan-forced airflow exceeding 0.5 m/s across the PLC, the Mitsubishi FX5U is the faster-processing choice with no thermal penalty. If ambient can reach 55°C and cooling is passive or minimal, the Siemens S7-1200 is the only controller that will stay online. The threshold is simple: ambient ≥50°C + no directed fan → Siemens. Otherwise, let the speed decide.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Siemens is a brand affiliated with this site; competitor names are used for identification only.