You need a controller for a 40‑axis packaging line and a supplier says “34 ns per instruction – fastest in class.” The other side says “85 ns, but the architecture doesn’t stall.” Which number actually determines your scan time, and which one is a marketing decoy? That’s what we tear down here.
The numbers. Mitsubishi FX5U basic instruction: ~34 ns. Siemens S7‑1200 (1214C standard): ~85 ns; the 1214C G2 variant runs ~40 ns. On the face of it, the FX5U is 2.5× faster than the standard 1214C and roughly on par with the G2. But a raw bit‑logic speed is only one term in the scan‑time equation.
Mechanism – why the ratio misleads. A PLC scan mixes bit logic, word moves, floating‑point math, and communication overhead. In typical machine logic, bit operations account for maybe 20–40 % of the instruction mix; the rest is data handling, indirect addressing, and function‑block calls. The FX5U’s 34 ns advantage shrinks when you factor in that the S7‑1200’s backplane and memory access latency don’t grow linearly with code size. More important: both CPUs run a deterministic scan, but the Siemens PLC architecture uses a separate communication pipeline (PROFINET) that does not steal cycles from the user program in the same way as the FX5U’s integrated Ethernet/RS‑485 stack. The real proportion: the raw bit speed difference of ~2.5× becomes roughly 1.2–1.4× in a mixed 60/40 load (word‑heavy).
Worked consequence. For a 5,000‑step mixed program (≈60 kbytes object code), the FX5U might complete the logic in about 170 µs versus the standard 1214C at roughly 340 µs — a 170 µs gap. In a machine that needs a 2 ms cycle, that 170 µs is ~8.5 % of the budget. You won’t feel it. But if you have 30,000 steps of pure bit logic, the gap swells to ~1 ms, which can force you into a sub‑2 ms scan – that’s where the FX5U’s raw speed pulls ahead.
Reversal. If your program is heavy on floating‑point, PID loops, or indirect addressing (common in packaging), the 34‑vs‑85 ns becomes irrelevant – the bottleneck shifts to the FPU and memory access, where both CPUs are roughly comparable. Also, if you exceed the FX5U’s program capacity of 64k steps, you tier up to a larger Mitsubishi CPU (e.g., R‑series), while the S7‑1200 can signal‑module expand without a CPU swap.
Table – raw I/O capacity vs. real expansion cost.
| Spec | Siemens S7‑1200 (1214C) | Mitsubishi FX5U (32‑point CPU) |
|---|---|---|
| On‑board DI/DO/AI | 14 DI / 10 DO / 2 AI | 16 DI / 16 DO (32‑point model) |
| Max local I/O (expandable) | ~284 (via signal modules) | 512 (via CC‑Link remote) |
| Built‑in analog | 2 AI (0–10 V, 14‑bit) | 2 AI (12‑bit) + 1 AO (12‑bit) |
| Communication ports (on CPU) | 1× PROFINET | 1× Ethernet + 1× RS‑485 |
Mechanism – where the real wattage hides. “Local I/O” is not free. Every expansion module adds backplane current draw and increases scan time because the CPU must update the process image. The S7‑1200’s 1214C has a 100‑kB work memory; the FX5U’s program capacity is up to 64k steps (~768 kB object code). But the critical number isn’t memory size – it’s how much I/O you can update within a single deterministic scan. The FX5U uses a fast internal bus (CC‑Link IE Field Basic) that adds ~0.3 µs per remote point, while the S7‑1200’s PROFINET update adds ~1–2 µs per slot depending on topology. For a machine with 256 mixed I/O, the additional scan tax from I/O update is about 0.5–0.8 ms on the FX5U vs. 1.2–1.6 ms on the S7‑1200. That’s where the FX5U’s aggregate throughput beats the S7‑1200 – not in raw logic, but in I/O scanning efficiency.
Worked consequence. For a 384‑point palletizer (240 DI, 144 DO), the FX5U’s scan tax from I/O update is ~0.6 ms, while the S7‑1200 is ~1.3 ms. Combined with a mid‑size logic of 8k steps, the total scan on FX5U is ~1.7 ms vs. S7‑1200 ~2.4 ms. That 0.7‑ms difference can mean the difference between a 2‑ms and a 2.5‑ms cycle – critical if your downstream conveyor timing demands ≤2 ms.
Reversal. If your application uses fewer than 100 I/O points, the I/O update tax is negligible on both platforms (
Numbers. S7‑1200 1214C: 100 kB integrated work memory. FX5U: “up to 64k steps” ≈ 768 kB object code. On paper, the FX5U has ~7.7× more memory. Yet many users hit a wall not on memory size but on memory segmentation: the FX5U splits code, labels, and comments into separate pools; the S7‑1200 uses a flat work memory model. In practice, the usable code space on the FX5U is roughly 400–500 kB for actual logic (the rest reserved for file registers and comments), while the S7‑1200’s 100 kB is all logic‑accessible. The proportion shrinks to about 4–5×, not 7.7×.
Mechanism – why code density flips the ratio. The S7‑1200’s KOP/FBD compiler produces more compact object code (about 1.2 bytes per instruction) versus the FX5U’s MELSEC instruction format (~2.0 bytes per instruction). So a 50‑kB Siemens program holds about 41,000 instructions; a 100‑kB Mitsubishi PLC program holds ~50,000 instructions – a ratio of 1.2×, not 2×. The real memory advantage of the FX5U is substantial only if you use heavy SFC (Sequential Function Chart) or ST (Structured Text) blocks, where the FX5U’s step capacity scales better.
Worked consequence. For a medium‑sized machine (40,000 instructions mixed), the S7‑1200 uses ~48–55 kB – well within its 100‑kB envelope. You have headroom for expansion. The FX5U uses ~80‑95 kB – also fine. The memory constraint only bites when you exceed 80,000 instructions; at that point the S7‑1200 forces a CPU upgrade (e.g., to 1215C or 1217C), while the FX5U can still fit in the same CPU. That’s a real win for Mitsubishi if you anticipate bloated code from multiple vendors.
Reversal. If your code is lean (engineered by one team using ST libraries), you will never approach 80k instructions. The S7‑1200’s smaller memory is irrelevant, and its better debug tooling (TIA Portal trace, watch tables) gives a faster development cycle. Also, the FX5U’s larger memory comes with longer backup/restore times – a hidden operations cost.
Base facts. S7‑1200: PROFINET (controller, RT). FX5U: Ethernet (TCP/IP, UDP, Modbus TCP) + RS‑485. Both can handle a few hundred bytes of cyclic data. The difference lies in how communication is scheduled. On the S7‑1200, the PROFINET stack runs in a separate kernel thread and does not block the user program. On the FX5U, Ethernet communication is processed in the user‑program time slice (unless you use the dedicated “communication buffer” mode, which still adds ~5–15 µs per packet).
Magnitude proportion. For a typical line with 10 remote I/O nodes (each 32 bytes I/O data), the S7‑1200 adds ~0.2 ms of background bus load; the FX5U adds ~0.5–0.8 ms of scan‑visible overhead. That 0.3–0.6 ms directly reduces your usable cycle budget. If your target scan is 2 ms, the FX5U’s comm overhead consumes 25–40 % of that budget; the S7‑1200 consumes only 10 %.
Worked consequence. On the same 40‑axis packaging line with 12 remote blocks, the S7‑1200 can sustain a 2‑ms scan with about 1.6 ms left for logic; the FX5U has only 1.2 ms left. That difference forces the FX5U user to either raise the target scan to 2.5 ms (slowing throughput) or strip logic – a direct tradeoff.
Reversal. If the network is lightly loaded (e.g., 2–3 remote I/O nodes), the overhead difference is
If your total scan budget is ≤ 1.5 ms and you need > 200 I/O points and > 30k instructions of mixed logic, the FX5U’s I/O scanning efficiency and raw bit speed give a measurable advantage – provided the communication load is ≤ 6 remote nodes.
If your scan budget is ≥ 2.5 ms, or you use fewer than 100 I/O, or you need integrated safety / complex motion (PTO, PID), the S7‑1200 delivers equal or better total throughput without the comm‑overhead tax, and with a more coherent toolchain.
If your code is > 80k instructions, the FX5U’s larger step memory is the only choice among these two – but consider that a 1217C (Siemens) also supports up to 150 kB work memory.
A user once picked the FX5U purely on 34‑ns speed for a high‑speed carton erector. The system had 14 remote valve banks (CC‑Link) and 60k instructions. The comm overhead ate up 1.1 ms, leaving only 0.9 ms for logic – the 34‑ns speed couldn’t compensate, and they had to drop two valve banks onto a second CPU. The S7‑1200 with PROFINET would have kept the entire line on one controller.
| Dimension | Siemens S7‑1200 (1214C) | Mitsubishi FX5U | Real‑world tiebreaker |
|---|---|---|---|
| Bit instr. speed (raw) | 85 ns / 40 ns (G2) | 34 ns | Only matters > 20k steps of pure bit logic; mixed code shrinks gap |
| I/O scan tax (256 pts) | ~1.3 ms | ~0.6 ms | FX5U wins for high I/O count; S7‑1200 wins for low I/O |
| Program memory (usable) | 100 kB flat | ~500 kB logic | FX5U only needed above ~60k instructions |
| Comm overhead (12 nodes) | ~0.2 ms (background) | ~0.7 ms (scan‑visible) | S7‑1200 leaves more budget for logic |
| Integrated motion / PID | PTO + PID on CPU | Positioning + HSC | S7‑1200 better for multi‑axis cam / PID loops |
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.