Allen-Bradley PLC vs Omron PLC: the spec that actually fails first

Popular industrial lore says a PLC fails either by blown power supply, a fried I/O card, or bit logic speed. In a head‑to‑head check of Allen‑Bradley CompactLogix 5380 and Omron Sysmac NX1P2, the spec that actually gives out first on a real machine is something else: memory granularity under motion + communications load. Not raw scan time, not I/O count — but the proportion of program memory eaten by axis configuration data, retentive variable arrays, and OPC‑UA server overhead. Here’s the teardown, dimension by dimension.

1. Program memory vs axis data – the proportion that bites

The CompactLogix 5380 (5069‑L306ER) arrives with 0.6 MB user memory . The Omron NX1P2‑9024DT brings 1.5 MB program memory plus 2 MB variable memory . On paper that looks like a 3× advantage for Omron PLC. But the deciding factor is how much of that memory gets consumed before you write one rung. A single EtherCAT axis in the Omron environment takes roughly 12–16 KB of variable mapping and configuration objects (about 1% of the variable block per axis) . For 8 axes that’s ~96–128 KB of variable space — still inside 2 MB, but the primary task cycle target (as low as 2 ms) demands that all axis data stay in contiguous, fast‑accessible main memory .

On the Allen‑Bradley PLC side, integrated motion over EtherNet/IP (CIP Drive) consumes configuration footprint proportional to the number of axes; with 32 axes possible on the 5380, each axis mapping consumes roughly 4–6 KB of the user memory pool (program + tag data) . A 16‑axis system uses about 64–96 KB of the 0.6 MB before any control logic — that’s 10–16 % of total memory burned on motion setup alone . The same proportion on the Omron (8 axes ~100 KB out of 1.5 MB + 2 MB) is ≈3 % of total memory. The worked consequence: a machine with sixteen servo axes plus a modest HMI data table (200 tags) will hit 80 % memory utilisation on the 5380 quickly; the same load sits at ~25 % on the NX1P2. When does this reverse? If you run a pure I/O‑replacement job with zero motion and fewer than 150 tags, the 5380’s 0.6 MB is perfectly comfortable — the Omron’s extra memory is just unused capacity with no speed difference.

2. OPC‑UA server – the hidden capacity tax

The Omron NX1P2 includes a built‑in OPC UA server with no additional hardware . The CompactLogix 5380 requires a separate PC‑based OPC server (like RSLinx Gateway or a third‑party OPC‑UA broker) unless you use the built‑in CIP‑Data Server; a true OPC UA server is not embedded . That’s a feature gap, but the failure mechanism is proportional: when OPC UA is activated on the NX1P2, it consumes about 0.3 MB of program memory and 0.2 MB of variable memory for the server stack and address space cache . That’s roughly 13 % of total memory. On the Allen‑Bradley, if you add an external OPC‑UA gateway, the memory tax is zero on the controller — but the gateway is an extra cost ($1,200–3,000) and a separate failure point .

Worked — a packaging line that requires OPC UA for MES connectivity: the NX1P2 loses ~0.5 MB total, leaving about 2 MB for logic and motion (still ample). The 5380 without OPC‑UA retains its full 0.6 MB, but you now depend on an external gateway’s reliability. The spec that fails first here is not the PLC’s memory, but the system‑level budget: if you embed OPC UA into the NX1P2, you must reserve ~25 % of variable memory; if you use an external gateway with the 5380, you risk a network break between PLC and MES. Reversal: if your operation has zero MES integration and no OPC UA requirement, the NX1P2’s built‑in server is dead weight that you can’t de‑allocate — it sits in ROM, occupying no dynamic memory, but you paid for the licence in the CPU cost. The 5380 is leaner in that scenario.

3. Communication node count – when the topology bends the cycle

CompactLogix 5380 supports up to 180 EtherNet/IP nodes (with the larger models) and Device Level Ring (DLR) . The NX1P2 supports up to 16 EtherCAT nodes and a separate EtherNet/IP subnet . These numbers are not directly comparable because they reflect different fieldbus technologies (EtherNet/IP vs EtherCAT). But the proportion of bandwidth used per node reveals the failure mode.

Assume a machine with 12 remote I/O blocks + 4 VSDs + 1 HMI (17 devices). On EtherCAT, the NX1P2 cycle time stays around 2–4 ms even with 20 nodes . On EtherNet/IP with the 5380, 17 nodes typically require 8–12 ms RPI (requested packet interval) to avoid congestion, but the controller’s backplane can handle 1 Gbps . The proportional failure point: if you have 50 devices, the NX1P2 cannot address them (16‑node limit), so it fails at the architecture level — you need an NX7 or bigger. The 5380 can scale to 150 nodes, but the RPI per node increases linearly. The worked result: a machine with exactly 15 distributed I/O blocks will run lean on either platform. The Omron’s limit is a hard ceiling; Allen‑Bradley’s limit is a soft soft‑real‑time ceiling that degrades gradually. Reversal: if your plant uses strict line‑synchronised motion across more than 10 servo axes, EtherCAT’s deterministic cycle (sub‑1 ms possible on larger Omron) is structurally superior to EtherNet/IP’s packet‑based cycle — the 5380 can handle 32 axes, but the jitter in CIP Sync is larger than EtherCAT’s distributed clocks. In that case the Omron architecture fails first only at the node count hard limit, not at performance.

Summary table – the proportion that matters

4. Temperature rise from power dissipation – a false alarm

CompactLogix 5380 dissipates 8.5 W max (29 BTU/hr) . Omron NX1P2 dissipates roughly 12 W (about 41 BTU/hr, derived from 24 V × 0.5 A typical) . Many integrators worry about “PLC hot spots” in small cabinets. But 12 W vs 8.5 W is a difference of 3.5 W — about the heat of a night‑light. The proportion that matters is enclosure size: a 300 mm × 300 mm × 200 mm steel cabinet with no fan rises roughly 3 °C from 10 W, 5 °C from 15 W. Neither unit fails thermally; the spec that actually fails first is the ambient rating of the adjacent 24 V power supply, not the PLC. So skip this dimension as a tie — the failure mode is not in the PLC.

Rule‑of‑thumb decision threshold

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.