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Choosing the wrong S7-1500 CPU is a mistake that costs weeks, not hours. Swapping a CPU mid-project means re-ordering hardware, updating the TIA Portal hardware configuration, re-downloading firmware, and sometimes re-wiring communication connections. Getting the selection right at the BOM stage eliminates all of that.
Below is a comparison of every standard S7-1500 CPU, from the entry-level 1511-1 to the 1518-4, covering the specs that drive selection decisions: work memory, instruction execution speed, communication interfaces, I/O capacity, and motion control capability. Compact CPU variants and a decision framework are included at the end.
Full S7-1500 CPU comparison table
The table below compares all six standard S7-1500 CPUs currently available. All figures are from Siemens datasheets for the current hardware generation.
| Specification | CPU 1511-1 PN | CPU 1513-1 PN | CPU 1515-2 PN | CPU 1516-3 PN/DP | CPU 1517-3 PN/DP | CPU 1518-4 PN/DP |
|---|---|---|---|---|---|---|
| Part Number | 6ES7511-1AK02-0AB0 | 6ES7513-1AL02-0AB0 | 6ES7515-2AM02-0AB0 | 6ES7516-3AP03-0AB0 | 6ES7517-3AP00-0AB0 | 6ES7518-4AP00-0AB0 |
| Work Memory (Program) | 150 KB | 300 KB | 500 KB | 750 KB | 2 MB | 6 MB |
| Work Memory (Data) | 1 MB | 1.5 MB | 3 MB | 5 MB | 8 MB | 20 MB |
| Bit Performance | 60 ns | 40 ns | 30 ns | 20 ns | 10 ns | 1 ns |
| Word Performance | 72 ns | 48 ns | 36 ns | 24 ns | 12 ns | 2 ns |
| Floating-Point Performance | 320 ns | 192 ns | 128 ns | 96 ns | 48 ns | 6 ns |
| PROFINET Ports | 2 (1 interface) | 2 (1 interface) | 2 (1 interface) | 4 (2 interfaces) | 4 (2 interfaces) | 6 (4 interfaces) |
| PROFIBUS DP | No | No | No | Yes (1 port) | Yes (1 port) | Yes (1 port) |
| Max I/O (Centralized) | Up to 32 modules | Up to 32 modules | Up to 32 modules | Up to 32 modules | Up to 32 modules | Up to 32 modules |
| Max PROFINET Devices | 32 | 64 | 128 | 128 | 256 | 256 |
| Motion Control Axes | — | — | — | — | — | — |
| OPC UA Server | Yes | Yes | Yes | Yes | Yes | Yes |
| Web Server | Yes | Yes | Yes | Yes | Yes | Yes |
| Price Tier | $ | $$ | $$$ | $$$ | $$$$ | $$$$$ |
Note on motion control: Standard S7-1500 CPUs do not include integrated motion control. If you need to control servo axes directly from the PLC, choose a Technology CPU (T-CPU) such as the 1511T, 1515T, 1516T, or 1517T. T-CPUs support up to 128 axes depending on the model.
CPU 1511-1 PN: entry-level controller
The CPU 1511-1 PN (6ES7511-1AK02-0AB0) is the lowest-cost entry into the S7-1500 platform. With 150 KB of program memory and a 60 ns bit instruction time, it handles straightforward machine control — conveyor sequencing, packaging lines, simple material handling — without overspending on capability you do not need.
Best fit: Single-machine applications with fewer than 256 I/O points, program sizes under 100 KB (leaving 30% headroom), and no PROFIBUS requirements. Typical applications include standalone packaging machines, small assembly cells, and simple conveyor systems.
When to skip it: If your application requires PROFIBUS DP connectivity, fast PID loops under 10 ms cycle time, or if you anticipate significant program growth beyond the 150 KB ceiling.
CPU 1513-1 PN: the mid-range pick
The CPU 1513-1 PN (6ES7513-1AL02-0AB0) doubles the program memory to 300 KB and improves bit performance to 40 ns. The 192 ns floating-point speed (versus 320 ns on the 1511) makes a tangible difference for applications running multiple PID loops or analog signal processing.
Best fit: Single-machine applications with moderate complexity — CNC feeders, automated assembly stations, labeling systems. The 300 KB program memory comfortably holds a typical HMI-connected machine program with alarming, recipe management, and 4-8 PID loops. This CPU is the most popular choice for OEMs building standardized machines.
When to skip it: If you need PROFIBUS, more than 64 PROFINET device connections, or if your compiled program exceeds 200 KB with growth expected.
CPU 1515-2 PN: mid-upper range without PROFIBUS
The CPU 1515-2 PN (6ES7515-2AM02-0AB0) steps up to 500 KB program memory and 30 ns bit performance. Its 3 MB data memory is double the 1513's, which matters for applications with large data blocks — recipe databases, trend logging, or multi-station interlocking tables.
Best fit: Multi-station production lines where complex interlocking logic pushes program sizes to 250-450 KB. Also a good choice when you run heavy data logging or recipe operations alongside the main machine program. The 128-device PROFINET limit supports larger distributed I/O networks.
When to skip it: If you have PROFIBUS devices in your network. The 1515 is PROFINET-only. For PROFIBUS, step up to the 1516.
CPU 1516-3 PN/DP: the PROFIBUS bridge
The CPU 1516-3 PN/DP (6ES7516-3AP03-0AB0) fills a specific gap: it is the lowest-cost S7-1500 CPU with an integrated PROFIBUS DP Master port. With 750 KB program memory, 5 MB data memory, and 20 ns bit performance, it also outperforms the 1515 in raw processing speed.
Best fit: Any application with existing PROFIBUS devices — legacy drives, remote I/O racks, or field instrumentation using PROFIBUS PA. The built-in DP Master eliminates the need for a separate CM 541-1 communication module (6ES7541-1AD00-0AB0), which costs nearly as much as the price difference between a 1515 and 1516.
Key advantage over the 1515: Two PROFINET interfaces (4 ports total) mean you can separate your PROFINET controller traffic from HMI/SCADA traffic on physically isolated subnets — a best practice for network segmentation.
CPU 1517-3 PN/DP: high performance
The CPU 1517-3 PN/DP (6ES7517-3AP00-0AB0) is built for applications where processing power is not negotiable. At 2 MB program memory and 10 ns bit performance, it handles large-scale production, complex batch control, and heavy data processing. The 48 ns floating-point time is 6.7x faster than the 1511.
Best fit: Programs exceeding 500 KB, sub-millisecond scan time requirements, multiple simultaneous PROFINET I/O controllers, or heavy OPC UA server loads from MES/SCADA systems.
CPU 1518-4 PN/DP: maximum performance
The CPU 1518-4 PN/DP (6ES7518-4AP00-0AB0) is the top of the S7-1500 standard range. With 6 MB program memory, 20 MB data memory, and 1 ns bit performance, it handles enterprise-scale architectures — thousands of I/O points, multiple PROFINET networks, and extensive OPC UA namespaces from a single controller.
Best fit: Large process plants or multi-line facilities. Its 6 PROFINET ports across 4 interfaces enable four physically isolated networks from one CPU — critical for redundancy and segmentation in high-availability applications.
Compact CPUs: 1511C and 1512C
For small machines where panel space is limited, Siemens offers two compact S7-1500 CPUs with built-in I/O:
| Specification | CPU 1511C-1 PN | CPU 1512C-1 PN |
|---|---|---|
| Part Number | 6ES7511-1CK01-0AB0 | 6ES7512-1CK01-0AB0 |
| Work Memory (Program) | 175 KB | 250 KB |
| Built-in Digital I/O | 16 DI / 16 DO | 32 DI / 32 DO |
| Built-in Analog I/O | 5 AI / 2 AO | 5 AI / 2 AO |
| High-Speed Counters | 6 | 6 |
| Bit Performance | 60 ns | 48 ns |
| Expandable | Yes (up to 30 modules) | Yes (up to 30 modules) |
Compact CPUs fit the S7-1500 platform into a smaller footprint. They work well for standalone machines where built-in I/O covers most requirements. You can still expand with additional S7-1500 I/O modules when needed.
Read more about all CPU variants in our S7-1500 Complete Buyer's Guide.
Decision framework: picking the right CPU
Use this step-by-step framework to narrow your selection:
Step 1: Estimate your program size
In TIA Portal, compile your project and check the "Resources" tab in CPU properties. For new projects, count function blocks and data blocks, estimate average sizes, and add 30% overhead. Single-machine programs typically fall between 50-200 KB; multi-station lines range from 200-800 KB.
Step 2: Check your PROFIBUS requirements
If you have any PROFIBUS devices — drives, remote I/O, field instruments — your minimum CPU is the 1516. Adding PROFIBUS to a 1511, 1513, or 1515 via a CM 541-1 costs nearly as much as upgrading to a 1516 and uses a module slot.
Step 3: Count your PROFINET devices
Each CPU has a maximum PROFINET device count. If your distributed I/O network (ET 200SP stations, HMIs, drives with PROFINET) exceeds the limit, you must step up to the next CPU tier.
Step 4: Evaluate scan time requirements
Two projects can have the same program size but vastly different performance needs. A bottling line at 1,200 bottles per minute needs faster scan times than a batch reactor that changes state every 30 seconds. If scan time matters, weight the bit/word/float performance columns heavily.
| Your Situation | Recommended CPU | Why |
|---|---|---|
| Single machine, simple logic, tight budget | CPU 1511-1 | 150 KB is sufficient; lowest cost entry |
| Single machine, moderate complexity, OEM standard | CPU 1513-1 | Best cost-per-KB; handles most single-machine programs |
| Multi-station line, PROFINET only | CPU 1515-2 | 500 KB + 3 MB data for complex interlocking and logging |
| Any system with PROFIBUS devices | CPU 1516-3 | Integrated DP Master saves slot + module cost |
| Large plant, multiple networks, heavy computation | CPU 1517-3 | 2 MB program + 8 MB data + 10 ns bit time |
| Enterprise-scale, multi-network, max I/O | CPU 1518-4 | 6 MB program, 4 PN interfaces, 1 ns bit time |
| Small machine, minimal panel space | CPU 1511C / 1512C | Built-in I/O reduces module count and wiring |
Common sizing mistakes to avoid
Under-buying memory
If your compiled project is 200 KB today, do not pick a CPU with 200 KB capacity. Scope creep, alarms, and change requests always push programs upward. Plan for at least 130% of your current compiled size.
Over-buying for "future-proofing"
A CPU 1517 for a simple palletizer is wasted money. If your program is under 200 KB with no PROFIBUS requirements, the 1513 will serve you well for the life of the machine. Spend the savings on better I/O modules or a larger HMI panel.
Forgetting network segmentation
The 1511, 1513, and 1515 have a single PROFINET interface. If you need to separate controller traffic from HMI/SCADA traffic on different subnets, you need the 1516 or higher — or an external managed switch with VLANs.
Ignoring floating-point speed
Bit performance gets the most attention, but many real-world programs spend more time on floating-point math — PID calculations, analog scaling, statistical computations. The gap between CPUs is even larger for float operations: the 1518 is 53x faster than the 1511 for floating-point instructions.
Frequently asked questions
Can I swap an S7-1500 CPU without rewiring the rack?
Yes. All standard S7-1500 CPUs use the same physical form factor and backplane connection. You can replace a CPU 1511 with a 1517 (or any other standard CPU) without touching the I/O wiring. Update the hardware configuration in TIA Portal, download to the new CPU, and the system is ready. Power supplies, I/O modules, and field wiring remain unchanged.
What is the difference between standard, F-CPU, and T-CPU variants?
Standard CPUs handle general-purpose automation. F-CPUs (fail-safe) add SIL 3 / PL e safety functions — they run both the standard program and a safety program for emergency stops, light curtains, and safety interlocks. T-CPUs (technology) add integrated motion control for servo and stepper drives, including cam, gear, and kinematics functions. TF-CPUs combine both safety and motion in one unit.
How do I check my program's memory usage in TIA Portal?
Compile your project in TIA Portal, then open the CPU properties and navigate to the "Resources" tab. It displays total work memory used versus available capacity, broken down into program memory and data memory. For projects still in the design phase, estimate by counting function blocks and data blocks, multiplying by their average size, and adding 30% overhead for runtime data and system blocks.
Is the CPU 1516 worth the upgrade from the 1515 just for PROFIBUS?
In most cases, yes. Adding PROFIBUS to a 1515 via a CM 541-1 communication module costs nearly as much as the price difference between the 1515 and 1516, and the 1516 also gives you 50% more program memory (750 KB vs 500 KB), faster execution (20 ns vs 30 ns bit time), and a second PROFINET interface for network segmentation. The 1516 is the better value whenever PROFIBUS is part of the architecture.
Related S7-1500 guides
- S7-1500 Part Number Decoder — learn how to read 6ES7 order numbers
- S7-1500 Safety (F-CPU) Guide — compare 1515F, 1516F, and 1517F fail-safe controllers
- S7-1500 vs S7-1200 — decide between the two Siemens PLC platforms
- S7-300 to S7-1500 Migration Guide — step-by-step upgrade path from legacy hardware
Browse S7-1500 CPUs
We stock all S7-1500 CPU variants — standard, compact, fail-safe, technology, and redundancy models — with a 2-year warranty. Browse our Siemens PLC Systems collection or request a quote for your specific project configuration.