S7-1200 vs. S7-1500: which CPU fits?
Short answer. The two families are not two speeds of the same thing. An S7-1200 CPU carries its digital inputs and outputs on the CPU itself and runs without a memory card; an S7-1500 CPU has no integrated I/O, needs a SIMATIC memory card, and is roughly two orders of magnitude quicker per instruction. If you are replacing a controller, the order number on the nameplate decides. If you are designing a new machine, the deciding questions are how many signals you terminate on the CPU and how many PROFINET ports you need.
Four SIMATIC CPUs, compared on what we can verify
Figures are taken from our own catalogue entries and from the Siemens TED product-data sheet for the exact order number. Blank means the attribute is not published for that article, not that it is zero.
| Order number | Type designation | Bit instr., typ. | Word instr., typ. | Floating point, typ. | Integrated DI / DQ | PROFINET interfaces | Ports | Memory card |
|---|---|---|---|---|---|---|---|---|
| 6ES7214-1AG40-0XB0 | CPU 1214C DC/DC/DC | 0.08 µs | 1.7 µs | 2.3 µs | 14 / 10 | 1 | 1 | optional |
| 6ES7215-1AG40-0XB0 | CPU 1215C DC/DC/DC | 0.08 µs | 1.7 µs | 2.3 µs | 14 / 10 | 1 | 2 | optional |
| 6ES7511-1AK02-0AB0 | CPU 1511-1 PN | 60 ns | 72 ns | 384 ns | none | 1 | 2 | required, max 32 GB |
| 6ES7515-2AM02-0AB0 | CPU 1515-2 PN | 30 ns | 36 ns | 192 ns | none | 2 | 2 | required, max 32 GB |
What the numbers actually decide
Instruction time is real, but rarely the bottleneck
A CPU 1515-2 PN is quoted at 30 ns per bit instruction against 0.08 µs for a CPU 1215C — a factor of roughly 27. In a scan-cycle-bound program that difference is large. In a machine whose cycle is set by PROFINET send times, fieldbus response or a motion block, the CPU is not what limits you, and the extra speed buys nothing.
Ports, not interfaces, are where the 1200s differ
CPU 1214C and CPU 1215C are identical on integrated I/O (14 DI, 10 DQ), on instruction time and on PROFINET interface count. The 1215C has two ports on that one interface instead of one. That is what lets it build a line topology without an external switch — often the actual reason a machine builder picked it.
The memory concept changes, not just the size
S7-1500 CPUs state that a SIMATIC memory card is required, with a maximum of 32 GB; S7-1200 CPUs run without a card. This is a programme-management difference, not a capacity difference, and it affects spare-part stocking: a 1500 CPU without its card is not a working spare.
Where this comparison stops
Program and work memory sizes, maximum configured signal modules, motion axes and fail-safe support are published by Siemens per firmware version in the product manual, not in the order data we can tie to an article number. We do not quote them from generic family pages, because a memory figure lifted from a brochure is exactly how a wrong spare-part decision starts.
If you need one of those figures for a specific order number, ask — we send the extract for that article.
Buying a CPU as a spare for a running machine
Order the number on the nameplate, unchanged. A CPU with more ports or a shorter instruction time is not a drop-in spare: the project, the module configuration and frequently the interface count were dimensioned to the original article, and the difference surfaces at commissioning rather than on the order.