Every Siemens S7-1200 manual says the CPU 1214C executes a bit instruction in ~85 ns. Every Mitsubishi FX5U datasheet says 34 ns. If you stop there, you think the FX5U is twice as fast. But the spec that actually fails first isn't the bit speed—it's the work memory ceiling, and it flips the magnitude of the entire performance picture. Let's tear it down.
The Siemens S7-1200 CPU 1214C offers 100 KB of integrated work memory. The Mitsubishi FX5U provides program capacity up to 64k steps—roughly 64 KB of user program memory (assuming ~1 byte per step for basic instructions). On paper they're close. In practice, the FX5U's 64k-step limit includes all program and function blocks combined, and once you fill it with motion, analog scaling, and communication handlers, you run out of space before your algorithm is done. The Siemens 100 KB is work memory (code + data), but TIA Portal compiles very efficiently; a typical medium control task with 4 axes of PTO motion, PID loops, and HMI data blocks eats about 40–50 KB of work memory. That leaves headroom. The FX5U, with its 64k-step cap, hits the wall when you add just two high-speed counters with match registers and a simple state machine.
Why this matters: Bit speed is a single-instruction metric. Real scan time is dominated by memory accesses, data type conversions, and network stack overhead. A CPU with 34 ns bit speed but only 64k steps of program space forces you to cram logic into fewer, less readable rungs. The Siemens PLC, with 100 KB and a slower bit time, runs the same task in fewer scan cycles because you write clean, modular code that doesn't need tricky indirection. The worked consequence: for any application with more than, say, 8 analog channels plus a simple state machine, the FX5U's memory runs full—and you either buy a larger model or delete diagnostics. The Mitsubishi PLC fan will say "but FX5U has up to 512 I/O with CC-Link"—true, but the CPU program memory doesn't scale. That's the hidden first-failure spec.
When it reverses: If your entire application fits in 30k steps—a single machine, 16 digital I/O, two analog inputs, no motion—the FX5U's 34 ns execution gives you genuinely faster cycle times. The Siemens's 85 ns bit time costs about 0.5 µs per rung in a typical 100-rung program, negligible for a 20 ms machine cycle. But if you take that 0.5 µs per rung and multiply by 1,200 rungs because you ran out of memory and had to use indirect addressing, the FX5U's speed advantage evaporates into complexity overhead.
| Spec | Siemens S7-1200 (1214C) | Mitsubishi FX5U (32MR/ES) |
|---|---|---|
| On-board digital I/O | 14 DI / 10 DO | 16 DI / 16 DO (typical) |
| On-board analog | 2 AI (0–10V, 4–20mA) | 2 AI (12-bit) + 1 AO (12-bit) |
| Expandability | Signal modules + SB + CM | Up to 512 I/O via CC-Link |
| Built-in comms | PROFINET (RJ45) | Ethernet + RS-485 |
The Mitsubishi FX5U packs more on-board I/O (32 vs 24) and includes a dedicated analog output. The Siemens gives only 2 AI, no AO on the CPU. On the surface, the FX5U wins the density game—more I/O per square inch, no extra modules. But the mechanism that fails first is the analog resolution versus updating speed. The FX5U's analog inputs are 12-bit; the S7-1200's are spec'd at 10-bit (default) but can be set to 12-bit in software under certain conditions. The Siemens PROFINET interface allows you to add remote analog modules (e.g., SM 1231) that run at 16-bit resolution and update at 100 Hz per channel. The FX5U's built-in analog is 12-bit and shares the CPU's scan cycle—meaning your analog value updates at the program scan rate, not at a dedicated converter rate. If you have a fast pressure control loop (say 10 ms cycle), the FX5U's 12-bit converter running at ~1 kHz may alias. The worked consequence: you think you saved a module slot, but you actually lost precision and update determinism. For flow or temperature loops that update every 100 ms, the difference is invisible. For servo-assisted press control, it's a failure mode.
When it reverses: If your application uses no analog or only uses slow sensors (temperature, level), the FX5U's 12-bit is sufficient and the built-in AO saves an extra module. The Mitsubishi wins on board cost and panel space.
Both PLCs come with a single built-in Ethernet port. The Siemens S7-1200 uses PROFINET RT with support for up to 16 IO devices; the FX5U supports Ethernet (SLMP, Modbus TCP) and RS-485. The spec that fails first here is the number of concurrent communication endpoints that don't degrade scan time. The S7-1200's PROFINET port is handled by a dedicated communication coprocessor (CP) integrated into the CPU; even with 8 IO devices and a HMI, the scan time increase is typically under 1 ms. The FX5U's Ethernet stack runs on the same CPU core that executes your ladder, so every Modbus TCP request or CC-Link IE Field Basic packet steals cycles from the main program. At 10 ms scan time you won't feel it. But if you have a 2 ms motion cycle (e.g., for a pick-and-place with 4 axes), the FX5U's communication overhead can add 0.5–1 ms per scan—that's a 25–50% increase. The Siemens keeps its scan jitter below 0.2 ms under the same load.
The worked consequence: for a packaging machine with one HMI, one VFD, and two I/O blocks, both work fine. For a multi-axis assembly cell with Ethernet/IP to two robots and a vision system, the FX5U's CPU can trip its watchdog. The Mitsubishi answer is to add an Ethernet module (FX5-ENET) which offloads the CPU, but that costs £300 and a slot. The Siemens answer is already built in: the PROFINET port is architecturally separate.
When it reverses: If your system uses no real-time motion over Ethernet and communicates via serial RS-485 to a VFD and a sensor, the FX5U's RS-485 port is native and very robust, while the S7-1200 needs an RS-485 module (CM 1241). The Siemens wins on motion, but the Mitsubishi wins on legacy serial integration without extra hardware.
TIA Portal versus GX Works3. Both are IEC 61131-3 compliant. The spec that fails first here isn't a performance number—it's the time to find a bug. GX Works3 stores programs in a project file that includes all comments, parameters, and configuration in a proprietary binary. TIA Portal stores each PLC in a separate device file, and the project-wide cross-reference can be filtered by hardware. The worked consequence: if you have a live machine with an intermittent fail-to-go-high on output Q0.2, in TIA Portal you open the cross-reference, filter by "Q0.2", and you see every instance in 1 click. In GX Works3, you need to run the "Device List" search, which takes 3–10 seconds for a 40k-step project. That's 5 minutes lost per day per engineer—roughly 20 hours per year per person. The Mitsubishi tool is not slow; it's just architecturally less efficient at cross-referencing.
When it reverses: If you are a lone programmer with a single machine type, you learn the GX Works3 search quirks and the overhead vanishes. For a team of five supporting 30 different machines, the TIA Portal cross-reference saves days.
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.