Is runtime under real load really the bottleneck? The one variable that separates Allen-Bradley from Omron — and the myth that keeps engineers ordering the wrong PLC.

Every plant-floor conversation about PLC runtime fixates on scan rate: "Omron PLC runs a primary task at 2 ms, so it must be faster under load." Or "Allen-Bradley CompactLogix 5380 has a 1 Gbps EtherNet/IP backbone, so it will never lag." Both statements are true in isolation, but neither answers the real question — what happens to that runtime when you actually add I/O, motion axes, and network nodes? The single variable that governs real-load runtime is not the raw cycle time — it's the ratio of memory bandwidth to the number of active objects (tasks, axes, nodes, safety routines). And on that ratio, the two families diverge sharply.

Myth #1: "Faster cycle time = faster under real load"

Omron's Sysmac NX1P2-9024DT quotes a primary task cycle as low as 2 ms. Allen-Bradley PLC's Micro850 does not publish a single-cycle number, but its architecture is based on a traditional scan loop; a typical empty scan for a Micro850 is roughly 3–4 ms (~10K steps executing at roughly 8–12 µs/step, derived from 10K program steps and 20 KB data memory). On paper, the Omron looks 1.5–2× faster. But that number only holds in a vacuum. The moment you add four axes of EtherCAT motion (Omron supports up to 8 axes), the NX1P2's primary task cycle will swell — not because the CPU is slow, but because the motion control loop demands deterministic synchronization across the EtherCAT bus, which consumes cycle time proportional to the number of axes and nodes. At four axes plus one EtherNet/IP scanner, the effective cycle often settles around 4–6 ms (illustrative, based on typical EtherCAT overhead). Meanwhile, the Micro850's PTO outputs (3 pulse-train outputs) are handled by dedicated hardware counters; the CPU only writes a start/stop command, and the scan loop barely stretches. For a machine with ≤3 servo axes and ≤16 I/O, the Micro850 can match or beat the NX1P2's real-world runtime because the axis count is below the Omron's advantage threshold. The myth collapses once you realize: raw cycle time is only meaningful when the CPU can keep up with the sum of its peripherals' demands. Reversal: if your application runs 6–8 servo axes with tight interpolation, the Omron's 2 ms baseline becomes decisive — the Micro850 has no on-board motion bus scalable beyond 3 PTO axes.

Myth #2: "More memory means faster execution under heavy code"

The Omron NX1P2-9024DT carries 1.5 MB program memory + 2 MB variable memory. The Allen-Bradley CompactLogix 5380 (5069-L306ER) starts at 0.6 MB user memory, expandable via SD card up to 32 GB. The myth says "more memory = more room for code = faster runtime." But the real bottleneck is not capacity — it's the memory bus and the way the CPU fetches instructions. The CompactLogix 5380 uses a dedicated high-speed backplane for Compact 5000 I/O, and its memory is structured as a multi-tier cache (roughly 60 ns bit-level execution for basic instructions, derived from typical 1 Gbps backplane latency). The Omron's memory is unified in a single address space, which simplifies programming but creates contention when large variable arrays (e.g., 2 MB of structured text arrays) are accessed repeatedly. In a scenario with 200+ local I/O points and a moderate ST program (say 8000 lines), the Allen-Bradley's memory architecture will complete a scan in roughly the same time as the Omron — despite having one-third the nominal program memory — because the fetch-and-execute pipeline does not stall on variable fetches. The real worked consequence: if your code is heavy on structured text with large data arrays (e.g., process recipes or lookup tables), the Omron's unified memory can actually feel slower under load because a single array access can block the task cycle. Conversely, if your code is 90% ladder logic with simple boolean operations, the Omron's larger program memory lets you pack more rungs without worrying about fragmentation — the CompactLogix's 0.6 MB base can fill up faster. The reversal: for applications with extreme program size (well above 1 MB of ST) and minimal array usage, the Omron wins — you won't hit the memory ceiling.

Myth #3: "Network speed determines runtime under distributed I/O"

Both platforms support 1 Gbps Ethernet. The CompactLogix 5380 embeds a dual-port 1 Gbps EtherNet/IP interface that can handle Device Level Ring (DLR) and up to 180 nodes. The Omron NX1P2 uses EtherCAT (100 Mbps) for motion + EtherNet/IP (100 Mbps) for I/O. The common belief: "1 Gbps is always faster than 100 Mbps, so the Allen-Bradley will have lower network-induced jitter." But network speed is the wrong variable. The real constraint is the total number of cyclic exchanges per task cycle. A CompactLogix 5380 managing 150 remote I/O nodes (each with 32 bytes of input + 32 bytes of output) must process ~9600 bytes of cyclic data per scan. Even at 1 Gbps, the CPU must unpack, validate, and map each packet into the controller tag database — a process that scales with node count, not line speed. The Omron, with its EtherCAT master limited to 16 nodes, handles far fewer exchanges per cycle, so its 100 Mbps bus is rarely saturated. The result: in a system with 60+ remote I/O nodes, the CompactLogix 5380's runtime will increase by 15–25% compared to a 16-node configuration (illustrative), while the Omron's runtime remains flat. The reversal is obvious: if your machine has ≤16 remote nodes and you need deterministic motion on the same cable, the Omron's EtherCAT gives you a single-bus solution that the CompactLogix cannot match without adding a separate motion network.

Takeaway: The myth that "faster cycle time = faster under load" is only half-true. The real arbiter is the ratio of memory bandwidth to active objects. For small-axis, moderate-I/O machines, the Allen-Bradley Micro850 or CompactLogix 5380 often holds runtime better because its memory architecture and dedicated I/O backplane avoid the contention that occurs when the Omron's unified bus is saturated. For multi-axis, low-node-count motion systems, the Omron's 2 ms baseline is unbeatable. Measure your axis count and remote node count — not the datasheet cycle time — and you will never over- or under-order again.

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.