First, a cold fact: you are looking at a 34 ns basic instruction vs an 85 ns basic instruction — and that 2.5× gap is the least reliable number on both data sheets. This tear-down dismantles what those numbers mean when your machine has to move 16 axes, handle a PID loop, and talk to an HMI at the same time.
The number. Mitsubishi FX5U claims a basic instruction time of ~34 ns; the Siemens S7-1200 (CPU 1214C) standard is ~85 ns. A naïve read: Mitsubishi PLC is 2.5× faster. The mechanism. That figure is measured on a single 16-bit Boolean operation with zero operand access overhead — no I/O mapping, no data movement. As soon as you add one MOV or a comparison (CMP), the cycle inflates by 8–12× on both architectures because the true bottleneck is memory throughput and operand staging, not the ALU. On a mixed workload (60% Boolean + 30% word moves + 10% math), the effective scan delta shrinks to roughly 1.4× in the Siemens PLC' favour because the S7-1200's single-cycle operand fetch via its backplane bus is more efficient per byte. The worked consequence. For a 3,000-step program (a typical conveyor sortation routine), the Mitsubishi scans at ~0.12 ms vs Siemens at ~0.21 ms — a 0.09 ms difference. That will never be the limiting factor on a machine with even a moderate mechanical process (20–50 ms cycle). The reversal. The gap matters only when your program exceeds ~15,000 steps and you are chasing sub-1 ms cycle times (high-speed pick-and-place, cam-driven electronic gearing). For 90% of industrial installations, the difference disappears into the servo update interval.
The number. Siemens S7-1200 1214C offers 100 KB integrated work memory; Mitsubishi FX5U lists program capacity up to 64k steps (where 1 step ≈ 4 bytes, giving ~256 KB). The mechanism. "Work memory" in Siemens is the runtime-executable portion of the program — all code and data must fit in this contiguous block. "64k steps" in Mitsubishi refers to the number of logical instructions stored in the program memory; there is a separate data memory (D-registers, 32k words) and system memory for parameters and device comments. The catch: the FX5U's step count includes all devices (contacts, coils, timers) as steps, whereas TIA Portal compacts the program into bytecode with separate data blocks. A function block with 20 parameters consumes roughly 1.2 KB on Siemens versus ~80 steps (approx 320 bytes) on Mitsubishi — but the Mitsubishi also consumes device memory for each instance. Worked consequence. For a machine with 30 pre-configured motion profiles and recipe arrays for 50 products (each with 200 floating-point parameters), the S7-1200 fills its 100 KB quickly — you hit the limit around 40% of the comparable step count. The FX5U's step-based model feels more generous, but you start running out of device memory (D-registers, link relays) earlier. The reversal. If your application is pure logic heavy (many state machines, few arrays), the FX5U's step model is genuinely more forgiving — you can pack 50,000 ladder contacts where the Siemens would run out of work memory at about 18,000 contacts. For data-heavy applications (recipes, recipes, arrays), the Siemens block model wins — a single 10 KB DB holds 1,250 float values, which would consume 5,000 D-registers (almost 15% of the FX5U's D-device limit).
The number. Siemens S7-1200 1214C comes with 14 DI / 10 DO / 2 AI on-board plus integrated PROFINET for motion (PTO). Mitsubishi FX5U offers up to 96 I/O on CPU (the 32-point variant has 16 DI / 16 DO) and built-in positioning via 4 high-speed counter / 4 PTO outputs. The mechanism. On the Siemens, motion is executed by the CPU's firmware running a PTO waveform generator clocked from the main PROFINET cycle — you configure a trajectory in TIA Portal, and the CPU outputs a pulse train directly from the integrated I/O. The Mitsubishi uses a dedicated hardware sequencer for positioning separate from the main CPU — the CPU sets target positions and speeds, then the hardware handles the pulse train without scan interference. Worked consequence. On a two-axis pick-and-place cycle (200 mm move, 1 m/s, 0.5 g accel), the Mitsubishi achieves 0.1 ms pulse jitter; the Siemens adds ~0.25 ms jitter due to scan-to-PROFINET interpolation. That 0.15 ms difference compounds over 100 cycles to a 15 ms positional uncertainty — enough to cause occasional collisions on a multi-pick tool with 0.5 mm clearance. The reversal. If your motion is ≤ 2 axes and you do not need sub-mm repeatability (e.g., conveyor divert gates, indexing tables), the Siemens jitter is irrelevant. But if you run 4+ axes or require true coordinated motion (electronically geared cams), the FX5U's dedicated positioning engine actually delivers lower peak torque variance because the CPU scan cannot disturb the pulse train — a rare case where the "smaller" architecture wins.
The number. Both Siemens SIMATIC S7-1200 (TIA Portal) and Mitsubishi MELSEC iQ-F (GX Works3) fully support IEC 61131-3 languages (LD, FBD, SFC, ST). Both are integrated development environments covering PLC, HMI, and drive configuration. The mechanism. The difference is in library management and cross-project reuse. TIA Portal uses a global library (with versioning and master copies) stored in a central .lib file; GX Works3 uses per-project label-based reuse with no global library manager across different PC projects. Worked consequence. For a 10-machine line each with 15 common function blocks (e.g., conveyor start, emergency stop, pressure regulation), the Siemens approach lets you update one library instance and recompile all projects in ~20 minutes. The Mitsubishi approach requires you to manually copy the FB into each project — a process that takes 3–4 hours and is error-prone (one missed update = field bug). The reversal. If you are a machine builder with fewer than 3 PLCs per year and no global library needs, GX Works3's lighter footprint (faster install, less RAM usage) is genuinely faster for standalone applications. The Mitsubishi environment also opens project files marginally faster (about 15 s vs 45 s on a typical laptop). But as soon as you cross the 5 PLC/year threshold, the TIA Portal library system saves more in engineering time than the hardware difference costs.
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. Siemens is a brand affiliated with this site; competitor names are used for identification only.