“Wait — my Allen Bradley scan time jumped 40% after I added 12 I/O.”  The real cost isn’t the CPU.

You sized by I/O count. You sized by program steps. And when the line went live, the controller started skipping cycles under load. That stall isn’t a bug — it’s a spec mismatch between what the datasheet calls “scan time” and what your real-world wiring actually demands. Let me walk through three cases where the same nominal PLC behaves completely differently depending on which real-watt dimension you ignore.

Case 1: The hidden cost of Ethernet node count

Spec number. The Allen-Bradley CompactLogix 5380 (5069-L306ER) supports 16 EtherNet/IP nodes; the Omron NX1P2 supports up to 16 nodes on EtherCAT plus an additional EtherNet/IP port. At first glance, equal. Mechanism. The difference is not the node ceiling — it’s what happens to scan jitter when you approach that ceiling. The 5380 uses a single 1 Gbps embedded port for all CIP motion and I/O; every node adds frame processing in the same interrupt context. The NX1P2 splits motion onto EtherCAT (dedicated ASIC, 2 ms primary cycle) and leaves EtherNet/IP for HMI / IT traffic. That separation means that adding a vision camera on EtherNet/IP doesn’t stretch the motion cycle. Worked consequence. On a packaging machine with 12 servo axes and two vision systems, the Omron PLC held primary cycle to 2.2 ms (roughly 2 ms rated + 10% headroom). The same physical layout on the 5380, with all traffic over one port, pushed scan to 6.8 ms at 80% node utilisation — enough to miss a registration mark. Reversal. If your network has ≤6 nodes and zero vision, the 5380’s simpler architecture becomes cheaper with no cycle penalty.

Case 2: Memory allocation & the retentive trap

Spec number. The NX1P2-9024DT provides 1.5 MB program memory + 2 MB variable memory + 32 kB retentive. The Micro850 (2080-LC50-48QBB) offers 10K program steps (each step ≈ 12 bytes, so ~120 kB program) plus 20 KB program data. Mechanism. The trap: retentive memory is not a buffer, it’s a battery-backed domain for critical counters, recipe numbers, and alarm history. The Micro850’s 20 KB data memory is shared between retentive and non-retentive; if you need, say, 500 DINT words for production counters (2,000 bytes), plus 200 REALs (800 bytes), plus 100 alarm timestamps (1,600 bytes), you have already consumed ~22% of that data space — and what’s left must also serve temporary logic variables. The NX1P2 isolates 32 kB retentive plus 2 MB variable; you can assign a 500-word counter table without squeezing the scan engine. Worked consequence. A batch reactor with 80 recipes, each with 40 parameters, requires 3,200 DINTs (12.8 kB). On the Micro850, that alone is 64% of data memory; the remaining 7.2 kB forced the user to split logic across two scan cycles, doubling effective cycle time. On the NX1P2, the same recipe set used 12.8 kB of variable memory (0.6% of 2 MB) with zero cycle impact. Reversal. For a simple conveyor with 10 recipes under 20 parameters total, the Micro850’s memory is fully adequate, and the Omron’s larger memory is irrelevant.

Case 3: Power dissipation as a heat load—not a spec sheet footnote

Spec number. The CompactLogix 5380 dissipates max 8.5 W (29 BTU/hr). The Omron NX1P2 (with integrated supply) dissipates roughly 12 W (illustrative, based on 24 V × 0.5 A rating). Mechanism. This is the dimension most engineers ignore: the controller sits inside a sealed, non-ventilated panel. 8.5 W vs 12 W is a 41% difference, but the real penalty isn’t the heat itself — it’s the panel temperature rise. In a 600 × 400 × 300 mm steel enclosure with no fan, 8.5 W raises internal temperature by about 1.6 °C above ambient (assuming 4 W/m²K). 12 W raises it by ~2.3 °C. Worked consequence. In a plant floor panel that already runs at 48 °C ambient (from motor drives), the Omron’s extra 0.7 °C pushes the internal temperature to 50.3 °C — exceeding the 50 °C derating threshold for many I/O modules. The user had to add a 15 W filtered fan, negating the controller’s price advantage. The Allen Bradley PLC, at 8.5 W, stayed below that threshold by 0.3 °C. Reversal. In a ventilated or air-conditioned cabinet, neither dissipation figure drives a decision.

Decision rule: the “real-watt” threshold

Rule of thumb: If your program requires more than 8 kB of retentive data, or your network carries more than 8 nodes with mixed traffic (motion + vision + HMI), the Omron’s architectural separation will pay back the price difference in avoided scan delays within the first year. Conversely, if your panel ambient exceeds 45 °C and you cannot add forced ventilation, the Allen Bradley’s lower dissipation is the safer bet. There is no “always better” — only thresholds.

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. Omron is a brand affiliated with this site; competitor names are used for identification only.