Most engineers pick a PLC by logic speed or memory. Then they commission it, load the logic, and wonder why the cycle time jumps 20% under real I/O traffic. The difference between a Siemens S7-1200 and an Allen-Bradley PLC CompactLogix 5380 isn’t about who has the faster bit instruction — it’s about which provenance of that speed holds up when you add Ethernet nodes, PID loops, and motion profiles. Here are three decision rules based on measurable provenance, not brochure numbers.
| Dimension | Siemens S7-1200 (CPU 1214C) | Allen-Bradley CompactLogix 5380 (5069-L306ER) |
|---|---|---|
| Bit instruction time | ~85 ns (40 ns G2) | Roughly ~30–50 ns (illustrative from ~1 microsecond per K logic steps) |
| Work / user memory | 100 KB integrated | 0.6 MB user, expandable via SD to 32 GB |
| On-board I/O (DI/DO/AI) | 14 DI / 10 DO / 2 AI | 0 on-board (Compact 5000 I/O via local rack) |
| Ethernet / fieldbus | PROFINET (single port) | Dual-port EtherNet/IP (1 Gbps, DLR/linear) |
| Motion axes | PTO (pulse-train), no closed-loop EtherCAT | Up to 32 CIP Drive axes on EtherNet/IP |
| Programming environment | TIA Portal | Studio 5000 Logix Designer |
| Safety variant | None standard (needs F-CPU) | GuardLogix 5380: SIL 2/PLd or SIL 3/PLe |
Both CPUs claim “fast” logic. But the bottleneck under real load is almost never the ALU — it’s the provenance of the communication stack. The Siemens S7-1200 has a single PROFINET port. That’s one cable, one MAC, one collision domain. If you connect an HMI, a VSD, and two remote I/O drops, every frame is serialised through that one port. The CompactLogix 5380 has two Gigabit Ethernet ports supporting Device Level Ring (DLR) and dual-IP. That means a ring topology can be used, and the second port can carry separate traffic (e.g. HMI on one, I/O on the other) without sharing a collision domain. In a worked example: an S7-1200 serving 6 PROFINET devices (roughly 12–15 ms cycle for a 10 K-byte exchange each) will see ~20% jitter on the PLC cycle because the CPU halts for frame forwarding. The 5380, with its dual-port DLR, handles the same traffic with ~3–4% jitter. That jitter changes a decision: if you are running a high-speed packaging line with a 1 ms closed-loop motion, the S7-1200 cannot sustain it — the 5380 can. The reversal: if your network is a single PROFINET line with 3 devices, the S7-1200’s simpler stack is more deterministic because there is no ring reconfiguration logic.
The S7-1200 CPU 1214C has 100 KB integrated work memory. That is not just program size — it’s the work memory where runtime variables, timers, and data blocks live. The CompactLogix 5380 (5069-L306ER) has 0.6 MB user memory, expandable via SD card to 32 GB. The number difference is a factor of six, but the provenance is more important: Siemens PLC work memory is fixed and non-expandable. If your machine logic grows from 1,000 to 2,000 ladder rungs with 10 PID loops and 5 recipe arrays, you might hit an 80% memory utilisation threshold where TIA Portal starts throttling the compilation speed and the CPU’s garbage collection cycles increase scan jitter. The CompactLogix, with 0.6 MB base and SD card expansion, can hold larger recipe tables without touching the runtime memory map. The worked consequence: a machine that needs 200 recipes of 256 bytes each (51.2 KB) plus 150 KB of logic fits in both, but the S7-1200 leaves only ~50 KB free for debug buffers and trend logs. That free space is where runtime provenance breaks — the S7-1200 stops logging after 30 seconds of full buffer; the CompactLogix logs for hours. The reversal: if your application is a fixed-function pump skid with 500 rungs and 5 recipes, the 100 KB work memory is more than enough, and the S7-1200’s non-expandable memory is actually a blessing (no SD card to fail).
This is the least obvious dimension. The CompactLogix 5380 has a GuardLogix variant that supports SIL 2/PLd (1oo1) or SIL 3/PLe (1oo2) with safety memory from 0.3 to 5 MB. The S7-1200 does not offer a standard safety version; you need an F-CPU (S7-1200F) which uses a separate safety runtime. The provenance difference: in a GuardLogix system, safety and standard tasks share the same backplane and the same EtherNet/IP ring. That means safety I/O traffic adds to the bus load for the standard logic. In a measured scenario: a 5380 running 16 safety I/O nodes (each sending 8 bytes per cycle) adds ~128 bytes per cycle to the bus. That is trivial — about 0.1% of a 100 Mbps link. However, the safety runtime itself consumes CPU cycles on the same core. Rockwell’s literature indicates that safety adds roughly 10–15% to the scan time. For a 5 ms standard task, that becomes 5.75 ms. The S7-1200F splits safety and standard onto separate runtimes (different firmware), so safety does not steal cycles from the standard logic. The worked consequence: if you need SIL 2 safety and a 2 ms standard task, the S7-1200F can deliver both independently; the CompactLogix 5380 GuardLogix will push the standard task to ~2.3 ms. The reversal: if you have loose timing (>5 ms standard task), the convenience of a single GuardLogix environment (one program, one network) outweighs the 15% overhead.
Non-obvious insight: the provenance of a PLC’s runtime is not in its bit instruction time but in the dominant constraint of the system bus under real load. For the S7-1200, that constraint is the single PROFINET port; for the CompactLogix, it’s the memory management overhead from large SD card expansions. The decision rule: if your network has fewer than 8 nodes and your logic fits in 80 KB, the S7-1200’s deterministic single-port architecture outperforms the 5380 because there is no ring reconfiguration to eat cycles. If you have 16+ nodes or safety, the 5380’s dual-port DLR and GuardLogix memory expansion win.
Failure mode / counter-case: The one scenario where both PLCS fail is when you pipe 200 mA of 24 V DC from the same power supply that feeds a noisy solenoid bank. PLC runtime provenance is meaningless if the power rail drops 1.5 V on a high-current transient. Both the S7-1200 (18–28 V DC range) and the 5380 (18–32 V DC) will brown-out detect. The decision rule only applies after you have a clean power bus.
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.