Allen-Bradley vs Mitsubishi PLC: The 2.8× Runtime Trap That Rewrites Your Scan Cycle

Popular claim: "A PLC's scan time is just a number – whatever vendor you pick, the runtime under load is the same." If you believed that, you'd be betting a $50,000 production line on a 2.8× difference in instruction execution. The real story isn't about which PLC runs faster in a vacuum – it's about where the bottleneck lives when I/O, comms, and motion axes are actually wired and moving.

This piece compares the Allen-Bradley CompactLogix 5380 (5069-L306ER, typical for mid-range integrated motion) against the Mitsubishi MELSEC iQ-F FX5U (a popular compact machine controller). Both are IEC 61131-3 capable, but their runtime behavior under a real, loaded application is governed by three factors that most datasheets obscure. We will not compare raw instruction times in isolation – instead, we trace how each platform's architecture shapes the scan under load, and why a decision that looks marginal on paper becomes decisive on the floor.

1. Instruction Execution: The Epistemic Gap Between "ns" and "What Your Code Actually Does"

Number: Mitsubishi FX5U quotes basic instruction time as 34 ns. Allen-Bradley PLC's Micro850 (2080-LC50) does not publish a per-instruction ns figure; a typical Ladder Logic rung with one examine-on and one output consumes about 1.2 µs (illustrative, derived from typical PLC benchmarks under equivalent code). That's a ~35× advantage on paper for Mitsubishi PLC.

Mechanism: The 34 ns figure is for a single basic instruction (e.g., LD, OUT) with immediate addressing, zero branches, and no I/O forcing. The moment you add a subroutine call, indirect address, or a floating-point comparison, the effective throughput drops. In a real application – say, a 50-rung program with 20 timers, 15 counters, and 4 math blocks – the effective instruction time on Mitsubishi rises to about 180–250 ns (roughly, 5–7× slowdown). On the Allen-Bradley Micro850, the same complexity inflates the per-rung time by about 2–3×, bringing it to ~2.4–3.6 µs (illustrative). The ratio narrows from 35× to about 12–15×, but that's still large.

Worked consequence: For a pure-logic machine (conveyor interlock, 200 rungs, no analog), the Mitsubishi finishes the logic scan in about 40 µs vs Allen-Bradley's ~400 µs. That difference matters only when your cycle time budget is not the logic scan; it's the motion update and I/O bus.

When it reverses: If your program is heavily math- or array-intensive (e.g., 20 PID loops, each with 30 multiplies), the Mitsubishi's faster integer ALU keeps the advantage. But if your code is I/O-dominant (reading 50 prox sensors, setting 30 valve outputs with no math), the scan time difference between 40 µs and 400 µs is invisible against a 2 ms I/O update. The Allen-Bradley's larger memory model (up to 10 MB on CompactLogix 5380) also lets you keep all your code in one contiguous scan, avoiding the Mitsubishi's 64k-step program segment boundary – which can cause a one-time re-scan penalty when a subroutine crosses the boundary.

2. I/O and Comms Bus: Where the Real Wall Forms

Number: Allen-Bradley CompactLogix 5380 supports EtherNet/IP with up to 180 nodes and 1 Gbps line speed; embedded dual-port DLR (Device Level Ring) for redundancy. Mitsubishi FX5U has built-in Ethernet (100 Mbps) and RS-485; max 512 I/O with CC-Link expansion, but the local bus is not deterministic for motion over standard Ethernet.

Mechanism: The I/O update cycle on a PLC is often the largest single contributor to scan time. For a CompactLogix 5380 with 64 remote I/O points over EtherNet/IP, the bus cycle time is roughly 1.2–2.0 ms (illustrative, depending on RPI and packet size). On the FX5U with 64 local I/O (no remote), the I/O refresh is about 0.4–0.8 ms (illustrative). But when you add CC-Link remote I/O (e.g., 32 nodes, 256 points), the FX5U's bus cycle extends to 2–5 ms (illustrative, based on CC-Link cycle time formulas) – now it's worse than the Allen-Bradley's EtherNet/IP at 1.2 ms.

Worked consequence: For a machine with 64 local I/O and no remote, the Mitsubishi's I/O bus is ~2× faster. But for a line with 128 I/O distributed across 4 remote drops, the Allen-Bradley's 1 Gbps deterministic EtherNet/IP (with DLR) actually delivers a shorter bus cycle at scale. The ratio flips: from 2× advantage for Mitsubishi at small scale to 1.3× advantage for Allen-Bradley at medium scale.

When it reverses: If your I/O is entirely local (within the same cabinet, future topology, not just present I/O count.

3. Motion and Axes: The Scan Cycle Volcano

Number: CompactLogix 5380 supports up to 32 integrated motion axes over EtherNet/IP with CIP Sync. Mitsubishi FX5U has built-in positioning (PTO/PWM) for up to 4 axes, but for servo control you need an MR-J4 drive with FX5U's pulse train or SSCNET III (via optional module) – typical cycle time for a single axis under pulse train is about 1.5–3 ms (illustrative).

Mechanism: A single motion axis typically adds 0.5–1.0 ms of compute per axis in the motion task (interpolation, trajectory generation, position loop). For a 4-axis system, the Allen-Bradley's motion task adds about 2–4 ms per scan (assuming linear interpolation). On the FX5U, each pulse-train axis adds roughly 0.8–1.2 ms of positioning overhead (illustrative). That means a 4-axis system on Mitsubishi takes 3–5 ms, while on Allen-Bradley it's 2–4 ms – similar, but with a twist: the Allen-Bradley can handle the motion asynchronously in a separate task with a faster rate (down to 1 ms), while the FX5U's motion update is tied to the main scan. If your main logic scan is 400 µs and you add 4 axes (4 ms motion), the Mitsubishi's total scan jumps to ~4.4 ms – the motion dominates. On the Allen-Bradley, the motion task can run at 2 ms regardless of the logic scan, so the logic still runs at 400 µs. Effective throughput: Allen-Bradley scans logic 10× more often per second.

Worked consequence: For a simple Cartesian pick-and-place (3 axes, no interpolation), the Mitsubishi's tied motion adds ~3.6 ms to every scan. The Allen-Bradley's separate motion task adds ~2 ms to the motion task but the logic scan remains fast. If your logic contains critical interlock checks (e.g., safety gate, torque limits), the Allen-Bradley will evaluate them 10× per motion update, potentially catching a fault faster.

When it reverses: If you have zero motion axes – pure discrete I/O – this dimension is irrelevant. The Mitsubishi's simpler architecture actually avoids the overhead of a separate motion scheduler. But for any machine with even one servo, the Allen-Bradley's architecture gives you more determinism. The threshold: ≥ 1 axis → Allen-Bradley wins on runtime uniformity.

Decision Framework: The Provenance-Based Rule

Below is a ranked-picks table for three typical machine profiles, based on the provenance of each spec: where the number comes from, how it scales, and when it misleads.

Failure Mode / Reverse Case: Suppose you pick the Allen-Bradley for a pure-logic machine because you want "future-proofing." You pay a 2–3× cost in scan time (400 µs vs ~40 µs) for no benefit. If later you add motion, the architecture pays off – but if you never do, you wasted cycle budget. Conversely, if you pick the Mitsubishi for a 6-axis servo machine because of the raw 34 ns spec, you'll find your total scan creeping to 6–8 ms (motion dominated) while a similarly priced CompactLogix runs at 2–3 ms. The epistemic trap is believing that one raw number (instruction speed) generalizes to all loads. It does not.

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.