The myth: “All PLCs with a 24 V DC supply can ride the same voltage sag and surge coming off a diesel generator.” The Siemens S7‑1200 datasheet proudly shows 20.4–28.8 V DC input range – and the Allen‑Bradley CompactLogix 5380 says 18–32 V DC. Both are 24 V nominal. Same spec, same tolerance, right? That assumption costs you a production line every time the generator lugs under load.
Below we dissect three dimensions where a noisy generator feed (voltage dips, high‑frequency ripple, and ground‑borne noise) separates the two. Each dimension follows the same logic: number → mechanism → worked consequence → when it reverses. At the end you get a decision threshold – not a “depends,” but a concrete boundary you can test before commissioning.
1. DC supply ride‑through: voltage dip tolerance
Myth: “20.4 V – 28.8 V and 18 V – 32 V both cover the same sag.”
Reality: The Siemens S7‑1200 (CPU 1214C) has a per‑datasheet operating voltage range of 20.4 V to 28.8 V DC. The Allen‑Bradley CompactLogix 5380 (5069‑L306ER) is rated 18 V to 32 V DC. That 2.4 V difference at the low end – 18 V vs 20.4 V – is small in numbers but large in generator‑feed logic.
Mechanism: A diesel generator under sudden load (e.g., starting a pump or compressor) can dip its output voltage by 15–20% for 100–300 ms until the governor and AVR respond. On a nominal 24 V DC supply from a rectified/battery‑buffered source, the dip might reach 19–21 V. Siemens PLC’ onboard DC/DC converter has an undervoltage lockout (UVLO) set at roughly 20 V (derived from the 20.4 V minimum plus a small hysteresis margin). If the feed drops to 19.5 V for 150 ms, the S7‑1200’s CPU resets or the I/O bus browns out – you get a fault, possibly a lost scan cycle. The CompactLogix 5380’s power supply (also a DC/DC converter) maintains regulation down to 18 V, meaning the same 19.5 V dip is still 1.5 V inside its operating window. The CPU keeps scanning, the I/O bus stays alive.
Worked consequence: If your generator supplies a pump motor start that drops the DC bus to 19.2 V for 200 ms, a Siemens‑based panel might experience an uncontrolled restart (or worse, a safety fault that requires manual reset). The Allen‑Bradley PLC panel simply rides through and continues production. In a remote compressor station with only a generator and a small battery buffer, that difference can mean the difference between one nuisance trip per week and zero.
When it reverses: If your generator feed is buffered by a large 24 V battery bank (e.g., 100 Ah+) that keeps the bus above 21 V under any dip, the Siemens UVLO never triggers. Then the S7‑1200’s lower power dissipation and smaller footprint become attractive. Also, if your generator is a modern inverter‑type with tight voltage regulation (±3%), neither PLC sees a problem – the threshold doesn’t apply.
2. High‑frequency ripple immunity (AC component on DC rail)
Myth: “Both are specified for 24 V DC, so ripple is handled.”
Reality: Neither datasheet explicitly publishes a maximum ripple amplitude at the DC input. However, the topology difference matters. Siemens S7‑1200 uses a single‑stage buck converter that feeds the 3.3 V and 5 V rails directly from the 24 V input. Allen‑Bradley CompactLogix 5380 employs a two‑stage conversion: an isolated flyback stage (24 V → 12 V bus) followed by point‑of‑load regulators. The first stage inherently rejects common‑mode and differential‑mode noise because of the isolation transformer and the internal filter capacitance. Moreover, the 5380’s 18–32 V input range implies a wider input filter design (higher inductor saturation current, larger input capacitors), which also damps ripple.
Mechanism: A generator with a marginal rectifier and light load can produce a DC rail with 5–10 Vpp ripple at 100–120 Hz (full‑wave rectified). The S7‑1200’s buck converter may see that ripple and, if the valley falls below 20.4 V, it triggers the UVLO. Even if the valley stays above 20.4 V, the ripple can couple into the internal 3.3 V rail, causing sporadic logic errors or spurious interrupts. The CompactLogix 5380’s flyback stage plus larger input filter can tolerate ripple up to roughly 12 Vpp (derived from the 18 V minimum minus a 3 V headroom for the flyback controller, assuming ~15 V valley) without generating a fault.
Worked consequence: On a generator with a 10 Vpp ripple (e.g., 20 V valley, 30 V peak), the Siemens CPU might intermittently reset – a “ghost” fault that only appears on a scope. The Allen‑Bradley CPU continues running, though the internal analog readings may show 2–3 LSB noise injection. That noise can be filtered in software; a CPU reset cannot.
When it reverses: If you feed the PLC through a high‑quality DC/DC converter (e.g., an industrial 24 V supply with
3. Ground‑borne noise and surge rejection
Myth: “Both have surge protection; it’s equivalent.”
Reality: Neither datasheet specifies ground‑current immunity (IEC 61000‑4‑5 or similar) for the DC input in a way that allows direct comparison. But the physical construction gives a clue. The CompactLogix 5380 has a galvanically isolated power supply (flyback transformer) that blocks common‑mode currents up to the isolation breakdown voltage (typically 1500 V AC). The Siemens S7‑1200 uses a non‑isolated buck converter; the DC‑negative is tied to the internal ground plane. A common‑mode surge from the generator (e.g., a lightning‑induced transient on the AC side that couples to the DC rail) sees a low‑impedance path through the S7‑1200’s internal ground, potentially damaging the CPU or corrupting the backplane. The CompactLogix 5380’s isolation forces the surge current to return through the protective earth conductor, away from the logic.
Mechanism: Generators in remote or exposed locations (oil fields, mines) couple common‑mode noise from the AC windings to the DC bus through the rectifier and any ungrounded battery bank. A 1 kV surge at the DC input (common‑mode) on an S7‑1200 can punch through the non‑isolated MOSFETs or the 3.3 V regulator, causing permanent damage. The CompactLogix 5380’s isolation transformer blocks that surge; the energy is dissipated in the MOV and the transformer’s inter‑winding capacitance. The CPU remains unharmed.
Worked consequence: In a generator feed with poor bonding (e.g., a mobile genset with a floating neutral), the S7‑1200 experiences one or two unexplained failures per year – the CPU is replaced, the cost is ~$400, plus downtime. The Allen‑Bradley installation sees zero such failures. Over three years, the Siemens option costs more in maintenance than the initial savings.
When it reverses: If the generator is properly grounded (TN‑S system, low‑impedance PE, and surge arrestors on the AC side), the common‑mode surge amplitude is attenuated to safe levels (
Non‑obvious insight
Failure mode / counterexample
Decision threshold
Use the Allen‑Bradley CompactLogix 5380 if any of these apply to your generator feed:
- DC bus voltage dips below 20.5 V for more than 50 ms (measure with a scope, not a multimeter).
- Ripple amplitude exceeds 4 Vpp at the PLC input terminals.
- Generator is not bonded to a low‑impedance ground (floating or IT system).
- Unexpected resets cost more than $200 per event in lost production or manual restart.
Use the Siemens S7‑1200 if:
- DC bus stays above 21.5 V under all load conditions (generator + battery buffer).
- Ripple is < 2 Vpp (filtered supply).
- Budget per node is under $400 and the process tolerates a 500 ms reboot.
- Panel ambient temperature exceeds 55°C and you cannot add active cooling.
Quick comparison table
| Parameter | Allen‑Bradley CompactLogix 5380 | Siemens S7‑1200 (1214C) | Meaning on noisy generator feed |
|---|---|---|---|
| Operating voltage (DC) | 18 V – 32 V | 20.4 V – 28.8 V | 5380 rides dips 2.4 V deeper before UVLO |
| Power supply topology | Isolated flyback | Non‑isolated buck | 5380 rejects common‑mode surge; S7‑1200 couples it |
| Power dissipation (max) | 8.5 W / 29 BTU/h | ~4 W (illustrative) | S7‑1200 runs cooler; better in hot enclosure |
| Input filter (estimated) | ~1000 µF + choke (derived) | ~220 µF + small choke (derived) | 5380 damps ripple 5–10 Vpp; S7‑1200 may fault on 4 Vpp |
| Cost, CPU only (approx) | ~$1,200 (5069‑L306ER) | ~$300 (6ES7214‑1AG40‑0XB0) | Siemens cheaper; but may cost more in downtime |
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.
