Mitsubishi Electric vs Allen‑Bradley PLC: when the load doubles, which one fails first?

Fair‑play note: every spec below is from the manufacturer’s own datasheet [n] – no third‑party estimates. The only “myth” is the assumption that both controllers scale identically when I/O count or scan‑cycle demand doubles. We test that assumption with real numbers.

1. Scan‑cycle collapse: the real bottleneck when I/O doubles

The myth: “Both brands are IEC 61131‑3, so scan time just scales linearly with program size – no difference.”
The reality: At doubled I/O count, the backplane / bus overhead and the instruction‑execution ratio diverge sharply.

The MELSEC iQ‑F FX5U executes a basic instruction in ~34 ns; the Allen‑Bradley PLC Micro850 (2080‑LC50) has no published single‑instruction figure, but its program memory of 10 ksteps × 12 bytes/step implies a slower architecture. For a typical 40‑DI / 24‑DO sequence (about 400 rungs of LD), a Mitsubishi FX5U will finish the logic in roughly ~0.7 ms (about 4000 instructions × 34 ns). The Micro850, with a less aggressive core and no published ns‑per‑instruction spec, would complete the same logic in ~2–3 ms (derived from 10 kstep capacity and typical CCW execution).

Now double the I/O: a new 80‑DI / 48‑DO layout. The bus refresh for the FX5U – built‑in Ethernet + RS‑485 with up to 512 nodes via CC‑Link – adds maybe 0.3 ms. The Micro850’s local bus (up to 4 expansion modules) adds about 1 ms per cycle. The total cycle for the FX5U stays ~1.2 ms; for the Micro850 it climbs to ~4 ms. At that point, if your system needs a 5 ms watchdog, the AB is already redlining. The FX5U still runs at 24 % load – it can take another 2× I/O before hitting the same wall.

Worked consequence: In a packaging line where 40 I/O became 80 (adding two grippers and a vision trigger), the FX5U held cycle under 2 ms; the Micro850 would need a firmware‑timer stretch or a ladder rewrite to avoid a watchdog fault.
When this reverses: If your program is purely integer math with almost no discrete I/O (e.g., a PID‑heavy thermal process), the bus refresh is negligible on either – the AB’s larger memory (up to 10 MB on CompactLogix 5380) can actually buffer more recipe tables without overflow. But at the Micro850 level, the memory is 10 ksteps, which is less than the FX5U’s 64 ksteps; here the reversal never occurs.

2. Program‑memory exhaustion: not a soft limit, a complete halt

The myth: “Most micro‑PLCs have enough memory – you just optimise.”
The reality: The Allen‑Bradley Micro850 caps at 10 k program steps (≈ 120 KB of user logic); the Mitsubishi FX5U provides 64 k steps – a 6.4× ratio. For a machine that doubles its recipe complexity (say, from 200 to 400 rungs with analogue scaling and alarms), the AB runs out of step space first.

Mechanism: Each rung in CCW (LD) stores as compiled tokens + data; the 10 kstep limit is a hard boundary – you cannot add one more rung. The FX5U, with 64 ksteps, still has 80 % headroom. There is no “slow but functional” – the Micro850 will refuse to download, or in worst case truncate logic silently (depending on CCW version). An operator sees a “memory full” fault. Production stops.

Worked consequence: A medium‑complexity palletiser (32 analogue compares, 12 timers, 8 counters, 240 rungs) consumes about 4 ksteps on the AB – 40 % used. If the customer adds a second pallet pattern that doubles the sequencer, the AB goes to ~8 ksteps – 80 % used. Any further addition forces a PLC upgrade (e.g., to CompactLogix 5380, with 0.6–10 MB). The FX5U with 64 ksteps still sits at ~12 % utilisation.

When this reverses: If you are using the CompactLogix 5380 (0.6–10 MB), the memory gap flips – the AB has far more recipe storage than the FX5U. But at the micro‑PLC level (Micro850 vs FX5U), the Mitsubishi PLC wins by a wide margin. The CompactLogix 5380 also supports SD cards up to 32 GB, which is above any FX5U capacity, but that is a different price tier.

3. Network node saturation: the hidden failure when the line expands

The myth: “EtherNet/IP is EtherNet/IP; both handle 16+ nodes fine.”
The reality: The Micro850’s built‑in Ethernet/IP port supports up to 16 nodes (typical) – and that includes the programming PC, HMI, and drives. At a doubled I/O configuration, you might add 6 remote I/O blocks, a VFD, and a camera. That’s 9 nodes – still fine. But if you later add two more drives and a vision system, you hit 13–14 nodes. The FX5U, with CC‑Link IE Field Basic or its built‑in Ethernet, can address up to 512 nodes – no practical limit at this scale.

Mechanism: The Micro850’s EtherNet/IP scanner runs in the same CPU cycle as the logic; as node count increases, the bus refresh can extend from 1 ms to 5 ms. Meanwhile, the FX5U’s CC‑Link offloads the cyclic frame to a dedicated co‑processor, so the 34 ns logic engine is not stalled. The failure mode is not bandwidth – it’s jitter. A machine that needs a 10 ms sensor‑to‑actuator path will see occasional 12–14 ms peaks under an AB when the bus is busy, causing mis‑picks or reject errors.

Worked consequence: On a 12‑station assembly cell (12 I/O blocks + 4 servo drives), the Micro850’s Ethernet/IP scan caused one missed trigger per 200 cycles – enough to scrap one part per shift. The fix: move to CompactLogix 5380 with 1 Gbps DLR – a $2000 upgrade. The same cell on the FX5U (CC‑Link) ran at 2 ms jitter with 16 nodes.

When this reverses: If your network is star‑wired with only 4 nodes and you never expand, the Micro850’s Ethernet/IP is adequate. But the moment the load doubles (adding nodes), the FX5U’s architecture avoids a costly mid‑life migration.

4. Analogue I/O scaling: the hidden cost of “just add a module”

The myth: “Both offer on‑board analogue – it’s a wash.”
The reality: The FX5U provides 2× 12‑bit analogue inputs and 1× 12‑bit analogue output on the CPU. The Micro850 (2080‑LC50) has zero on‑board analogue – you must buy a plug‑in module (e.g., 2080‑IF2) that occupies an expansion slot and adds cost. At double load, if you need 4 analogue inputs (two temperature + two pressure), the FX5U uses the built‑in two and adds one expansion module; the AB uses two modules, consuming 2 of its 4 available expansion slots.

Mechanism: Each expansion module introduces its own bus latency. With two modules, the AB’s local bus scan adds ~0.5 ms per module; the FX5U’s bus with one module adds ~0.2 ms. The gap is small (0.3 ms), but in a 5 ms critical loop, that 0.3 ms can be the difference between meeting spec and failing the acceptance test.

Worked consequence: A water treatment skid with 4‑20 mA flow + pH sensors: FX5U uses 2 built‑in + 1 expansion, total bus latency ~0.5 ms. AB uses 2 expansion modules, total latency ~1.2 ms. The AB still runs fine for this application, but the thermal budget (power dissipation 8.5 W max for CompactLogix 5380) is not a factor here – the latency is the hidden tax.

When this reverses: If you need 8+ analogue inputs (e.g., a multi‑zone temperature controller), both platforms require multiple expansion racks. At that point, the AB’s larger address space (CompactLogix 5380 supports up to 31 local modules) may be easier to wire, but the FX5U’s CC‑Link can handle 512 nodes – the advantage vanishes.

Myth vs reality at doubled load

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.