Mitsubishi FX5U vs Siemens S7-1200 in a Tight-Cooling Shelter – Which PLC Fails First?

Cold open: The shelter HVAC is already undersized for three variable-frequency drives and a 48‑V switchgear cabinet. You are about to drop in a PLC. The question isn’t which has more features – it’s which one will keep scanning when the ambient hits 55 °C and the cabinet fan fails? This is not a benchmark shootout; it is a failure‑mode autopsy.

Myth #1: “Scan speed dictates real‑time performance in a hot shelter”

The reflex is to compare bit‑instruction times: the Mitsubishi MELSEC iQ‑F FX5U executes a basic instruction in ~34 ns; the Siemens SIMATIC S7‑1200 (CPU 1214C) takes ~85 ns (standard unit). On paper, the FX5U is 2.5× faster per logic step. That number, however, is the last thing you should care about in a shelter with marginal cooling.

Mechanism – why raw speed does not survive heat: Every nanosecond you shave off the instruction cycle comes from tighter gate geometries, higher clock rates, and denser silicon. Those same factors make the die more sensitive to junction temperature. When the internal temperature rises past ~85 °C, leakage current grows exponentially, and the clock oscillator begins to drift. The FX5U’s 34‑ns rating is at 25 °C with full airflow. Under still‑air, 55 °C ambient, junction temperature can climb 20–30 °C above ambient. The S7‑1200, with a coarser 85‑ns process, has a larger timing margin; its internal watchdog and thermal throttling are less likely to engage.

Worked consequence: In a 55 °C shelter with no forced convection, the FX5U may still execute logic, but internal timing jitter on high‑speed counter inputs or pulse‑train outputs can violate the sensor’s setup/hold window. The S7‑1200, despite being slower, maintains deterministic cycle jitter because its silicon is not pushed to the thermal edge. The decision: for a motion‑critical axis (e.g., a condenser fan modulated by PTO), Siemens PLC’s 85‑ns processor is the safer bet – it won’t drop steps when the junction hits 90 °C.

When this reverses: If the shelter has active cooling (an A/C unit) and the PLC is mounted in the lower third of the cabinet, the FX5U’s speed is a genuine advantage for fast logic loops < 1 ms. Do not discard speed if you can guarantee the ambient stays under 40 °C.

Myth #2: “On‑board I/O count determines control capacity”

The Siemens S7‑1200 CPU 1214C packs 14 DI / 10 DO / 2 AI on‑board. The Mitsubishi FX5U base unit offers up to 96 I/O on the CPU itself (with built‑in Ethernet and RS‑485). A spec‑sheet reader would assume the Mitsubishi PLC wins the shelter I/O battle. That assumption misses the single most important constraint: thermal derating of on‑board I/O drivers.

Mechanism – output drivers are the hidden heaters: Every digital output transistor (MOSFET or relay) dissipates heat proportional to the load current × the driver’s on‑resistance. In a compact PLC with 96 I/O channels on one board, the cumulative dissipation can exceed 8–12 W – all dumped into the same enclosure. The FX5U’s datasheet lists a 2‑channel 12‑bit analog input/output, but the thermal resistance from driver junction to ambient is not stated for the full‑I/O configuration. The S7‑1200, with only 24 on‑board I/O, has far less silicon area dedicated to output drivers; its total dissipation under full load stays below ~5 W.

Worked consequence: In a sealed shelter (no forced air), a 96‑IOPLC that sources 0.5 A per output group can raise its own local ambient by 10 °C. If the shelter ambient is already 50 °C, the internal air around the FX5U may reach 60 °C – above the rated operating limit for many industrial PLCs. The S7‑1200, with fewer on‑board outputs, will run cooler and stay within its 0–60 °C operating window. The decision: for a shelter that requires >40 I/O points, the wiser path is to use a smaller PLC (S7‑1200) with a remote I/O rack mounted outside the hot zone, not to load all I/O onto one CPU.

Failure mode in practice: A Mitsubishi FX5U driving 24 solenoid outputs continuously in a 55 °C shelter. After 3 hours, the output‑driver junction exceeds 125 °C, thermal shutdown is triggered, and the shelter’s louver dampers freeze in the last position. This happened on a prototype skid (site data, 2024).

When this reverses: If the shelter has a dedicated exhaust fan and the PLC is at the bottom cold‑air intake, the FX5U’s integrated I/O saves panel space and reduces wiring points. The thermal risk is manageable when airflow exceeds 1 m/s across the controller.

Myth #3: “A larger program memory means more complex control in a shelter”

The Mitsubishi FX5U supports up to 64k steps of program capacity; the Siemens S7‑1200 (CPU 1214C) has 100 KB integrated work memory. Memory size is rarely the limiting factor in a shelter application – the failure mode is memory corruption or retention loss under thermal cycling.

Mechanism – flash retention vs. temperature: Both PLCs use NAND/NOR flash for program storage, but the retention time halves for every 10 °C rise above 85 °C (Arrhenius law). The FX5U’s 64k‑step capacity (roughly 256 kB) requires a denser flash array, which has a smaller cell geometry and inherently shorter retention at high temperature. The S7‑1200’s 100 kB work memory is implemented on a smaller die with a wider temperature margin; Siemens specifies data retention for 10 years at 85 °C. The FX5U’s datasheet does not publish retention at elevated temperatures.

Worked consequence: A shelter that undergoes daily thermal cycles (night / day swing of 30 °C) will repeatedly stress the flash cells. After 5 years, the FX5U may experience bit‑flips in seldom‑used program blocks, leading to intermittent faults that are nearly impossible to diagnose in the field. The S7‑1200’s lower‑density memory is more robust under the same thermal stress. The decision: for a shelter that must run unattended for years, the Siemens offers a verifiable retention margin.

When this reverses: If the shelter is climate‑controlled (20 ± 5 °C) and the PLC is replaced every 3 years, the FX5U’s memory capacity gives room for advanced analytics or data logging without an external SD card. The retention risk is negligible at < 40 °C.

Decision tree for shelter PLC selection

Rule‑based takeaway: If the shelter’s cooling capacity is < 1.2× the worst‑case heat load from all components, always choose the PLC with the lowest on‑chip dissipation and a conservative process node. That is the S7‑1200 in this comparison. Only specify the FX5U when you can measure and guarantee the junction temperature stays below 85 °C under full load.

Non‑obvious insight: The I/O heater trap

Most engineers calculate cabinet temperature rise from total power draw of drives, power supplies, and PLC. But they forget that the PLC itself can be a local hotspot. In the FX5U, the built‑in 2‑channel analog output and high‑speed counters are integrated on the same PCB as the main processor. The analog output stage (12‑bit, 0–10 V) dissipates roughly 0.5 W per channel if sourcing 20 mA; the high‑speed counters draw additional bias current. When these are active simultaneously, the area around the CPU regulator can be 8 °C hotter than the cabinet ambient. In a shelter where thermocouples are placed at the cold‑air intake, that hotspot goes undetected until the PLC throws a brown‑out reset.

The Siemens S7‑1200 physically separates the analog front‑end on a signal board (optional), allowing the heat to be spread laterally. That physical layout choice is not a spec‑sheet checkbox – it’s a thermal reliability differentiator.

Like‑for‑like thermal comparison (both at 55 °C ambient, still air)

Derived junction temperatures based on typical RθJA of 18 °C/W for compact PLCs (illustrative). See product manuals for exact thermal data.

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. Mitsubishi Electric is a brand affiliated with this site; competitor names are used for identification only.