“That PLC cycles fast—but can you keep that speed when the plant floor hits 50 °C and the panel is sealed?”

You picked a PLC based on raw scan speed. The MELSEC iQ-F FX5U advertises a basic instruction time of ~34 ns, and that number catches your eye. But here is the cost of that choice: you install it in a 45 °C electrical room with a 10 °C temperature rise inside the panel, and the controller's internal junction temperature inches toward its derating curve. The 34 ns you measured on the bench starts to stretch—and worse, the I/O module you added for the analog loop now runs at half the sample rate because the bus timing compensates for heat drift. The efficiency you thought you bought evaporates. This is the eligibility gate: not whether a PLC can execute an instruction fast, but whether it can sustain that performance under the conditions your plant actually imposes. Let's walk the three dimensions that decide whether a controller keeps its speed or loses it.

1. Thermal stability of the logic engine

The Mitsubishi FX5U runs its basic instruction at 34 ns. The Allen-Bradley CompactLogix 5380, in a comparable mid-range controller (5069-L306ER), does not publish a raw bit time, but its scan cycle is dominated by the user memory size and the number of axes—typical ladder scan times for a 5000-step program land around 0.5–1.5 ms, which is an order of magnitude slower per cycle than the FX5U. On paper, the Mitsubishi PLC wins. But the FX5U's 34 ns is measured at 25 °C with the CPU at rated load and no adjacent heat sources. The CompactLogix 5380 is specified for operation up to 60 °C ambient with no derating of performance until the internal junction exceeds 90 °C. In a real panel—say 40 °C ambient, with a 24 VDC supply dissipating 30 W and a VFD 15 cm away—the FX5U's internal temperature can rise 10–15 °C above ambient. At 55 °C internal, Mitsubishi's datasheet recommends a 20 % derating of the I/O load and a reduction in high-speed counter frequency. That derating is not about breaking—it is about the controller slowing its internal bus timing to maintain data integrity. The 34 ns instruction time becomes roughly 41 ns (about 20 % longer) under the temperature-compensated clock. Meanwhile, the CompactLogix 5380, with its 0.6 MB user memory and 8.5 W dissipation, runs at the same scan rate at 55 °C as it does at 25 °C because its silicon is rated for industrial junction temperatures and the firmware does not throttle. Worked consequence: If you have a high-speed packaging machine that requires a consistent 2 ms loop for a registration sensor, the FX5U might drift to 2.4 ms on a hot afternoon, causing a reject every 50 packages. The CompactLogix 5380 will hold its 2 ms loop. When this flips: If your panel is actively cooled (air-conditioned to 25 °C) and your machine runs in a clean, climate-controlled lab, the FX5U's speed advantage is real and you can keep it. But in a non-conditioned plant floor environment, the thermal eligibility gate eliminates that advantage.

2. I/O bus timing under load vs. expansion overhead

The FX5U supports up to 512 I/O points via CC-Link, and its local bus on the FX5U-32MR/ES handles 16 on-board I/O with a typical update rate of 1 ms for the local rack. The CompactLogix 5380, with its 1 Gbps EtherNet/IP backplane and local expansion chassis, can update 256 digital I/O in under 0.5 ms. Here is the mechanism: the Mitsubishi uses a proprietary high-speed bus that is clocked at 50 MHz, but each remote CC-Link module adds a fixed 0.3 ms of propagation delay per hop. The Allen-Bradley PLC uses a deterministic CIP Sync ring (DLR) that maintains jitter under 10 µs across 50 nodes. When you add 64 remote I/O points via CC-Link, the FX5U's worst-case I/O scan jumps to 1.6 ms. The CompactLogix 5380, adding the same 64 points via EtherNet/IP in a linear topology, sees 0.6 ms. Worked consequence: In a machine that requires a safety-rated input to stop a motor within 20 ms (e.g., a press brake), the FX5U's I/O bus overhead consumes 8 % of your budget, leaving only 18.4 ms for the safety relay and contactor—marginal. The CompactLogix 5380 leaves 19.4 ms, which is more comfortable. When this flips: If your I/O is mostly local (within one chassis) and you use only the built-in analog and digital points, the FX5U's bus is fast enough—its 1 ms local update is fine for conveyors and batch processes. But the more you expand, the more the Mitsubishi loses its speed edge.

3. Real-world power efficiency: the heat you actually have to remove

This is a dimension where the raw spec misleads. The FX5U CPU module dissipates about 12 W at full load. The CompactLogix 5380 (5069-L306ER) dissipates 8.5 W. Both are modest. But the Mitsubishi's expansion modules—especially the FX5-16EYT/ES (16-point output) and the FX5-4AD (analog input)—each add 3–5 W. A typical 48-point configuration with 4 expansion modules runs at 12 + 4×4 = 28 W. The Allen-Bradley Compact 5000 I/O modules (e.g., 5069-IB16) dissipate about 2.5 W each, so the same 48-point configuration runs at 8.5 + 4×2.5 = 18.5 W. That is a 34 % lower heat load. In a sealed panel with no fan, that difference means about 5 °C lower internal temperature. Worked consequence: Over a one-year continuous run in a 40 °C ambient, the Mitsubishi configuration adds about 87 kWh of extra heat energy that must be removed by the panel cooler (assuming a COP of 2.5, that is 35 kWh of compressor work—about $4.20 at $0.12/kWh). Not huge, but the real cost is the thermal headroom: the Mitsubishi's higher dissipation pushes it closer to the derating threshold, which then impacts the first two dimensions. When this flips: If you install the PLC in a ventilated enclosure with a fan and a 15 °C ambient, the heat difference is irrelevant. The efficiency you keep is then purely the logic speed, which Mitsubishi wins.

The non-obvious insight: the Mitsubishi is faster on the bench but slower in the heat

The typical buyer looks at cycle time and stops. But the PLC's internal clock is temperature-compensated; the silicon's propagation delay increases with junction temperature by about 0.1 % per °C. A 20 °C rise adds 2 % to instruction time. Combine that with the I/O bus overhead from expansion and the higher dissipation of the Mitsubishi system, and the 34 ns advantage can shrink to a 10 ns advantage—or disappear entirely if the FX5U's I/O derating kicks in and forces a slower bus clock. The Allen-Bradley, with its wider operating range, lower dissipation, and deterministic network, keeps its performance curve flat across the plant floor conditions that cause the Mitsubishi to sag.

Failure mode: when the eligibility gate shuts on the Mitsubishi

The worst case: a panel with a 40 °C ambient, a VFD within 30 cm, a 64-point I/O expansion, and a 24 VDC power supply running at 80 % load. The FX5U's internal temperature hits 55 °C, the instruction time drifts 5–10 % longer, and the I/O bus adds 0.3 ms of delay. The machine cycle time that was 10 ms on the bench becomes 11.2 ms on the floor. The operator increases speed to compensate, but the registration sensor misfires once every 200 cycles. Scrap rate climbs. The Mitsubishi is not broken—it is just not eligible for that environment.

Rule-based takeaway

Comparison snapshot

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.