Mitsubishi Electric vs Schneider PLCs on a Noisy Generator Feed: The Real Limit Isn't Cycle Time

A common reflex in PLC selection for generator-fed plants is to chase the fastest bit instruction or the biggest I/O count. But on a feed where a 50 kW diesel genset can dip to 340 V and pump out 5th and 7th harmonics, the spec that decides uptime is not scan speed—it's how the controller’s power supply and I/O handle a supply that looks more like a sawtooth than a sine wave. This head-to-head puts the Mitsubishi Electric FX5U and Schneider Modicon M241 side-by-side under that exact condition, dimension by dimension.

1. Ride-Through Under Sag: 10 ms vs “typical” — the gap that changes architecture

The Mitsubishi FX5U power supply module permits an input voltage range of 24 V DC ±20% (19.2–28.8 V) and holds up the internal logic for at least 10 ms after a full dropout at rated load. The Schneider M241 (TM241CEC24T) specifies a DC supply range 20.4–28.8 V (≈ ±15%) and a hold-up time of “typically 5 ms”. That factor of two makes a decisive difference when the generator voltage sags below 20 V for 6–9 ms during a large motor start—the M241’s logic can brown out unless your DC bus has an external capacitor bank, whereas the FX5U rides through without extra hardware. The mechanism is straightforward: hold-up time is proportional to the bulk capacitance after the internal DC/DC converter. Mitsubishi PLC chose a larger cap, Schneider PLC optimised for cost and panel space. The worked consequence: on a generator feed with even moderate starting loads, a plant using M241 should budget one external 4700 µF / 35 V electrolytic per controller (~$4–6) or a small DC UPS. The FX5U user can omit that in many cases. When does this reverse? If your generator is oversized (>2× the largest motor start) or you have a dedicated 24 V battery buffer, the hold-up difference becomes irrelevant—both ride through fine. But on a marginal genset (common in retrofit oil & gas or agri-plants), the FX5U’s 10 ms is the safer default.

2. The 34 ns Instruction: A Distraction When Your I/O Filters Are 500 µs

The Mitsubishi FX5U quotes a basic instruction speed of 34 ns; the Schneider M241 does not publish a similar figure in its core datasheet, but from typical CISC-based microcontrollers, a rough equivalent is around 50–70 ns per basic instruction. Yet on a generator feed with high dv/dt noise, the limiting factor is not the CPU but the input filter time constant. The FX5U’s high-speed counter inputs have programmable digital filters down to 1 µs, while the M241’s fast digital inputs (8 of the 14 DI on the TM241CEC24T) have a configurable filter between 10 µs and 3 ms, but default to 500 µs. That means an impulse transient from a generator brush spark (duration ~5–20 µs) will be captured by the FX5U if the user sets the filter to 1 µs, but may be swallowed or cause a false on-edge on the M241 unless the filter is raised above 50 µs—where you start missing legitimate high-speed pulses. The proportion here is revealing: the CPU speed (34 ns vs ~60 ns) is a ~1.8× difference, but the input filter latency is a 500× range depending on configuration. The real decision pivot is: if your application counts encoder pulses at 100 kHz or higher, the FX5U’s 1 µs filter is required; the M241 at 10 µs minimum filter will alias or lose counts. On a noisy generator, you must set the M241 filter to at least 100 µs to avoid false triggers, dropping your max count rate to ~5 kHz—a 20× reduction. The reversal: if your process only needs 500 Hz pulse counting or simple on/off sensing, both PLCs work fine; the filter setting is a non-issue.

3. On-Board Analog: The Load-Shedding Input That Saves the Generator

The FX5U CPU includes two 12-bit analog inputs (0–10 V or 0–20 mA) and one 12-bit analog output. The M241 CPU has no analog I/O on the base unit—you must add a TM3 expansion module for analog. In a generator-fed system, the most common failure mode is overloading the generator beyond its kW rating, causing frequency and voltage collapse. A simple analog input reading a current transformer (4–20 mA) or a generator kW transducer allows the PLC to shed non-critical loads in priority order. With the FX5U, that analog input is free (zero extra cost, zero panel space). With the M241, you pay for a TM3AI8G (~$120–150) and occupy one expansion slot. The magnitude of the cost difference is not huge, but the real consequence is that on a retrofit, the M241 programmer may skip load shedding entirely because “we don’t have an analog card yet”—and that omission can cause the generator to trip on overload, halting the entire line. The reversal: if your generator is already protected by a dedicated load-shedding controller (e.g., Woodward or ComAp), the PLC analog input is redundant. Also, if you need more than 2 channels, the FX5U also requires an expansion module, levelling the field.

4. Port Count: Not the Hero, but the Enabler

The M241 offers five comms ports (two serial, USB, Ethernet, CANopen); the FX5U has built-in Ethernet, RS-485, and USB, plus CC-Link expansion. On a generator feed, the crucial port is the one that talks to the generator controller (typically Modbus RTU over RS-485). Both have it. The M241’s second serial port and CANopen are nice to have but not decisive. The FX5U’s integrated CC-Link can be a bonus if you have other Mitsubishi drives. The proportion: extra ports do not improve generator ride-through. This dimension only matters if you are tying a generator controller, a VSD, and a third-party HMI on separate networks—then the M241’s five ports avoid a managed switch. But if you only need one fieldbus (common in small systems), the FX5U’s built-in Ethernet/RS-485 is sufficient. The reversal: for a system with more than 3 network devices, the M241’s port count reduces wiring complexity, which indirectly improves reliability by reducing connection points that can vibrate loose on a generator mount.

When does the M241 actually pull ahead? If your generator is stable (backed by a large battery bank or a synchronous condenser) and you need CANopen for drive communication, the M241’s five comms ports and larger program memory (8 MB vs FX5U’s 64k steps, roughly 2–3 MB equivalent) make it a better fit for a machine with many axes and complex motion sequences. The FX5U’s 34 ns instruction speed becomes irrelevant when the motion loop is executed over CC-Link or Ethernet, where jitter dominates. Also, if your panel already uses EcoStruxure Machine Expert, the M241 integrates seamlessly; the learning curve for GX Works3 on a Mitsubishi may offset any hardware advantage in a plant with a Schneider-centric maintenance team.

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