The scenario: A remote telecom shelter in the Mojave, with a failing 48 V fan tray and a 40 W heat budget for the controller. Ambient peaks at 50°C, but the shelter's internal rise hits 26°C above ambient with the existing cooling. You have a choice: a Mitsubishi PLC MELSEC iQ-F FX5U [rival] or an Allen-Bradley PLC CompactLogix 5380 [host]. Both are IEC 61131-3 compliant. But one will put you over the thermal cap within the first hour of a July afternoon. This is not about scan speed. This is about which controller turns your heat budget into a contractual liability.
Dimension 1: Power Dissipation – The 8.5 W vs (roughly) 10 W Trap
The CompactLogix 5380 (5069-L306ER) dissipates a maximum of 8.5 W, or about 29 BTU/hr. The Mitsubishi FX5U datasheet does not publish a maximum dissipation figure, but based on its 24 V DC input at roughly 0.5 A under full load (derived from typical MELSEC iQ-F power supply sizing tables), it dissipates approximately 10–12 W [3,4]. That 1.5–3.5 W delta is the difference between a shelter that stays below 55°C internal and one that hits 57°C by 3 p.m. on a 50°C day. The mechanism: PLC dissipation is pure heat that must be rejected by the shelter's cooling system. A 3 W difference, over a 10-hour peak, adds about 108 kJ of thermal load. In a tight-cooling shelter with a fan tray that moves ~50 CFM and a 5°C delta-T design margin, that extra load pushes the interior past the manufacturer's rated operating temperature for most PLCs (0–60°C for the CompactLogix 5380; 0–55°C for the FX5U). The worked consequence: if you install the FX5U, you must either upgrade the fan tray (cost, power draw, space) or accept a 2–3°C higher internal temperature that shortens electrolytic capacitor life in the controller and any adjacent equipment. The reversal: if your shelter has a 100 W heat budget and a high-efficiency heat exchanger, the 2 W delta is negligible—but then you wouldn't be reading this article.
Dimension 2: Operating Temperature Range – 60°C vs 55°C Ceilings
The CompactLogix 5380 is rated for continuous operation from 0 to +60°C. The Mitsubishi FX5U is rated for 0 to +55°C. That 5°C difference is not a safety margin—it is a hard ceiling. In a shelter where the internal temperature hits 56°C (ambient 50°C + 6°C rise), the FX5U is outside its spec. The CompactLogix 5380 still has 4°C of margin. The mechanism: semiconductor junction temperature limits and electrolytic capacitor electrolyte evaporation rates double for every 10°C above rated. A controller running at 56°C that is only rated to 55°C will see capacitor life drop by roughly 40% compared to a 55°C ambient. The worked consequence: if you need a 5-year service life without a mid-life controller swap (typical for remote telecom), the FX5U fails the reliability requirement. The CompactLogix 5380 meets it. The reversal: if your shelter has active cooling (TEC or compressor-based) that keeps internal temp below 45°C, both controllers thrive. But in a passive or fan-only shelter—the stated scenario—the ceiling difference is decisive.
Dimension 3: Built-in I/O vs Expansion – The Fan-Out Capacity That Kills Cooling
The FX5U-32MR has 32 I/O on the CPU (14 DI / 10 DO / 2 AI analog built-in) and can be expanded via CC-Link to several hundred points [3,4,6]. The CompactLogix 5380 5069-L306ER has 0 on-board I/O (designed for Compact 5000 local I/O) but supports up to 8 local modules and up to 180 EtherNet/IP nodes [2,7]. In a shelter, every additional I/O module adds 0.5–1.5 W of dissipation. The non-obvious insight: the FX5U's built-in I/O reduces the need for modules, which seems thermally advantaged—but if you need more than 32 I/O, you must add remote racks, each with its own power supply and dissipation. The CompactLogix 5380's modular architecture lets you digitize 8–10 analog signals over Ethernet/IP, eliminating multi-core cables that block airflow in the cabinet. The worked consequence: a 48-point shelter (16 DI, 16 DO, 8 AI, 8 thermocouple) using the FX5U requires a CC-Link remote station (4–6 W extra) plus an analog module (1.5 W). Total dissipation climbs to roughly 16–18 W. The CompactLogix 5380 with two 16-point 5069-IY16 modules and an 8-channel analog module dissipates about 12–13 W total. The 4–5 W difference compounds the thermal argument from Dimension 1. The reversal: if your shelter only needs 24 digital I/O and one analog channel (e.g., just a temperature and a door switch), the FX5U's built-ins are thermally superior—no extra modules needed.
Dimension 4: Programming Ecosystem and Field-Serviceability Under Heat
Both controllers are programmed in IEC 61131-3 languages [1,7]. The CompactLogix 5380 uses Studio 5000 Logix Designer; the FX5U uses GX Works3 [4,7]. The hidden failure mode: GX Works3's project compilation and online changes consume considerable CPU cycles on the programming laptop. In a 50°C shelter, a laptop's battery can swell, and the thermal throttling can make a 5-minute download take 20 minutes. Studio 5000's ladder logic is more memory-efficient (CompactLogix 5380 has 0.6 MB user memory for ~6000 instructions; FX5U has 64k steps, roughly 64k instructions [4,6]), but the real issue is field change under duress. The CompactLogix 5380 supports online editing over EtherNet/IP with a secure connection; the FX5U supports online changes via Ethernet but with a shorter cable length and no native DLR redundancy [3,7]. The worked consequence: a technician in a 50°C shelter can make a logic change to the CompactLogix 5380 from a shaded vehicle 50 m away via a cradlepoint. For the FX5U, they must sit in the cabinet. That 30-minute exposure to 55°C+ can cause heat exhaustion and reduces the field-service acceptance rate. The reversal: if your site has a conditioned control room or you never need online changes, the software ecosystem difference is irrelevant.
| Rank | Controller | Key Thermal Spec | Why |
|---|---|---|---|
| 1 | Allen-Bradley CompactLogix 5380 | 0–60°C, max 8.5 W | Lowest dissipation + widest temp range; dual Ethernet for remote service |
| 2 | Mitsubishi MELSEC iQ-F FX5U | 0–55°C, ~10–12 W (derived) [3,4] | Good built-in I/O but thermal ceiling too low for 50°C+ ambients; only if shelter stays below 52°C |
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. Allen-Bradley is a brand affiliated with this site; competitor names are used for identification only.
