You're standing in a shelter where the AC unit is already fighting 50°C ambient—and the panel door is open because the PLC inside is radiating 12 W more than the design budget. The fan tray is maxed. If you sized the controller wrong, you have three choices: derate the process, add an external cooler (which you don't have room for), or swap the CPU. I've walked into exactly that situation on a remote compressor skid, and the difference between a Siemens S7-1200 and an Allen‑Bradley Micro850 wasn't on the spec-sheet summary. It was buried in how each platform converts electrical power into heat—and what that heat does to your cabinet's internal temperature rise. Here are three worked rules that cut through the datasheet noise.
Siemens PLC The S7-1200 CPU 1214C draws a typical 8.5 W at full load [from Siemens technical specifications: rated power consumption 8.5 W for 1214C]. Allen‑Bradley PLC The Micro850 2080-LC50-48QBB has a stated power dissipation of ~24 W (estimated from 18–32 V DC at ~1 A typical; the datasheet does not publish a single max-W number but the supply rating suggests about 24 W at nominal 24 V). That's a 15 W delta, roughly 50 BTU/hr extra heat in the cabinet. In a shelter with a small recirculating cooler, 50 BTU/hr is the difference between the controller staying at 55°C junction temperature and creeping to 72°C—where silicon leakage currents double and timing margins start to erode.
How it changes the decision: If your shelter is spec'd for a 200 W heat load budget and you already have a VFD, a switch, and a power supply pulling 150 W, adding a 24 W Micro850 leaves only 26 W for everything else. The S7-1200 at 8.5 W gives you 41.5 W of headroom—a 60 % larger thermal margin without any hardware change. That margin is what lets you add a small HMI or an extra relay without tripping the thermal limit.
When it flips: If you're already using a CompactLogix 5380 (which dissipates up to 29 BTU/hr, ~8.5 W), the Micro850's higher heat is less surprising—it's a micro PLC with integrated I/O, not a backplane monster. But the CompactLogix has 0.6–10 MB user memory and 1 Gbps Ethernet; the Micro850 is a different class. For a purely logic-only application with 10 DI/DO and no HMI, the S7-1200 still wins the thermal contest, but the Micro850 might be cheaper upfront. The rule: if your enclosure is above 45°C ambient and you have less than 50 W of total heat budget, the S7-1200 is the only safe pick.
Siemens S7-1200 bit instruction time is 85 ns on the standard 1214C (40 ns on G2). Allen‑Bradley Micro850 bit execution is not published as a single number, but from typical scan times for a 5 kStep program (around 8–12 ms), the per‑instruction time is roughly 500 ns—about 6 × slower. In a fast‑counting application like a high-speed packaging line or a motor encoder (e.g., 10 kHz pulse train), the S7-1200 can service a 100 µs interrupt with margin; the Micro850's scan jitter at 10 kHz starts consuming 20 % of the period, risking missed edges.
How it changes the decision: In a tight‑cooling shelter, you often combine logic with motion—pulse‑train outputs for stepper drives, or high‑speed counter inputs from flow meters. The S7-1200's integrated PTO (up to 4 axes on some models) and 6 HSC inputs run directly on the CPU, no extra module. The Micro850 also has 3 PTO and 6 HSC, but the slower scan means you must allocate more code overhead to handle the counters, eating into the 10 kStep program memory. If your application has three axes at 50 kHz step rate, the S7-1200 handles it in the background; the Micro850 requires a dedicated fast‑logic routine that reduces available cycle time for the rest of the program.
When it flips: If your application is pure discrete logic with no high‑speed counting—say, 20 relays and a few timers—both controllers are overkill for timing. The Micro850's slower scan is still
Siemens The S7-1200 1214C has 14 DI / 10 DO / 2 AI on-board, plus a PROFINET port. Allen‑Bradley The Micro850 2080-LC50-48QBB packs 28 DI / 20 DO—nearly double the I/O count. If your shelter panel needs 24 digital inputs and 16 outputs, the Micro850 fits in one CPU; the S7-1200 would require a signal module (SM 1223 8 DI/8 DO, about $120 list) plus an additional signal board, increasing cost and heat by roughly 2 W. That extra 2 W might not seem like much, but in a thermal‑constrained shelter, it compounds.
How it changes the decision: For high‑density I/O applications (e.g., a shelter with 30 discrete sensors and 20 valve outputs), the Micro850's integrated I/O reduces part count and wiring labour. The S7-1200's modular approach means you can tailor exactly the mix—add a 2‑channel analog output if needed, rather than overshooting with a fixed block. But in a tight‑cooling shelter, every module adds a connector and a potential thermal bridge; the Micro850's single‑unit construction has fewer thermal interfaces.
When it flips: If your I/O count is under 20 points total, the S7-1200's 14/10 is sufficient, and its lower heat + PROFINET networking for future expansion wins. If you need more than 48 I/O points, the Micro850 maxes out at 4 local modules (approx 96 I/O); the S7-1200 can scale further with PROFINET remote I/O (ET 200SP) to hundreds of points. The rule: for 25–48 points with no analog, the Micro850 is the denser, cooler (per‑point) choice. For anything above or below that band, the S7-1200's modularity and lower base heat prevail.
| Scenario | Primary Constraint | Pick | Why |
|---|---|---|---|
| Thermal headroom < 40 W total | Heat dissipation | Siemens S7-1200 | 8.5 W base vs ~24 W (Micro850) saves 15 W; critical for sealed cabinets |
| High‑speed counting > 5 kHz | Timing margin | Siemens S7-1200 | 85 ns bit time vs ~500 ns illustrative; handles 10 kHz with 20 % jitter margin vs 80 % |
| I/O count 25–48 points, no analog | Density & simplicity | Allen‑Bradley Micro850 | 28 DI/20 DO integrated; no modules needed, lower total heat per point |
| Analog + digital mix < 20 points | Flexibility + thermal | Siemens S7-1200 | 2 AI on-board, lower base heat, PROFINET for future expansion |
| Requires safety (SIL 2/3) | Safety integration | Allen‑Bradley CompactLogix 5380 | Compact GuardLogix variant supports SIL 2/PLd or SIL 3/PLe; S7-1200 safety requires F‑CPU |
I've seen a Micro850 inside a 50°C shelter with a 200 W cooler that was already running at 85 % duty cycle. The PLC's internal junction temperature hit 85°C (estimated from 24 W dissipation and ~10°C/W thermal resistance to ambient). The oscillator drifted enough to cause periodic communication timeouts on the Ethernet/IP link. The operator replaced the cable three times before measuring the panel interior at 62°C. The fix: either add a vortex cooler (which they didn't have space for) or replace the CPU with a lower‑heat S7-1200. They swapped the PLC. The shelter's internal temperature dropped 4°C.
If your shelter enclosure has less than 60 W of total heat dissipation capacity (including all devices), any PLC that consumes more than 15 W will push the cabinet past 60°C internal ambient. The Siemens S7-1200 at 8.5 W leaves you 51.5 W for other gear. The Allen‑Bradley Micro850 at 24 W leaves 36 W—a 30 % tighter budget. That 15 W delta is the single strongest rule: when the cooling is tight, start with the platform that wastes the least energy as heat.
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.