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If your machine has an emergency stop circuit, a safety light curtain, or safe speed monitoring, you need a safety-rated PLC. Siemens S7-1500 F-CPUs (Fail-Safe CPUs) integrate SIL 3 / PL e safety functions directly into the standard CPU — no separate safety relay system required.
This guide compares the S7-1500 F-CPU models we stock, explains how safety programming works in TIA Portal, and helps you choose the right F-CPU for your safety application.
What makes an F-CPU different from a standard CPU?
An S7-1500 F-CPU runs two programs simultaneously:
- Standard program: Normal automation logic (same as any S7-1500 CPU)
- Safety program: Certified fail-safe logic (SIL 3 / PL e per IEC 62061 and ISO 13849)
The safety program runs in a separate, redundantly executed partition within the CPU. If the CPU detects any inconsistency between the redundant calculations, it forces all safety-related outputs to their safe state (typically OFF).
Key benefit: One CPU handles both standard automation and safety — fewer components, less wiring, smaller panels, and unified diagnostics in TIA Portal.
F-CPU comparison table
| Spec | CPU 1511F-1 PN | CPU 1513F-1 PN | CPU 1515F-2 PN | CPU 1516F-3 PN/DP | CPU 1517F-3 PN/DP |
|---|---|---|---|---|---|
| Part Number | 6ES7511-1FK02-0AB0 | 6ES7513-1FL02-0AB0 | 6ES7515-2FN03-0AB0 | 6ES7516-3FN02-0AB0 | 6ES7517-3FP00-0AB0 |
| Work Memory | 225 KB | 450 KB | 750 KB | 1.5 MB | 3 MB |
| Safety Work Memory | Part of total | Part of total | Part of total | Part of total | Part of total |
| Bit Performance | 60 ns | 40 ns | 30 ns | 20 ns | 10 ns |
| PROFINET Ports | 2 | 2 | 2 | 2 | 2 |
| PROFIBUS DP | No | No | No | Yes | Yes |
| PROFIsafe | Yes | Yes | Yes | Yes | Yes |
| Safety Certification | SIL 3 / PL e | SIL 3 / PL e | SIL 3 / PL e | SIL 3 / PL e | SIL 3 / PL e |
Note: F-CPUs have more work memory than their standard equivalents (e.g., CPU 1511F has 225 KB vs 150 KB for CPU 1511) because the safety program consumes additional memory for redundant execution and safety data blocks.
F-CPU vs. standard CPU + safety relay: why integrate?
| Aspect | Standard CPU + Safety Relays | F-CPU (Integrated Safety) |
|---|---|---|
| E-stop monitoring | Wired through safety relay, separate from PLC | Processed in CPU safety program, logged in diagnostics |
| Panel space | Safety relays + DIN rail + wiring = significant space | No additional hardware beyond F-I/O modules |
| Diagnostics | Limited — safety relay LEDs only | Full diagnostics in TIA Portal, HMI, and web server |
| Flexibility | Hardwired logic — changes require rewiring | Software-based — safety logic changes via TIA Portal |
| Safe motion | Requires separate safe motion controller | Integrated SLS, SLP, SOS, SS1, SS2 in the F-CPU |
| Documentation | Separate safety circuit diagrams | Safety program is part of the TIA Portal project |
For simple E-stop applications on a single machine, a safety relay can be sufficient. But the moment you need safe speed monitoring, multiple safety zones, or diagnostic visibility in your HMI, the F-CPU approach saves money and reduces complexity.
Choosing the right F-CPU
CPU 1511F: entry-level safety
The CPU 1511F-1 PN is the most affordable F-CPU. It is suitable for machines with a small safety program — typically a few E-stops, light curtains, and safety door switches.
Best for: Single-machine safety with under 10 safety I/O points and a simple safety program.
CPU 1513F: the OEM workhorse
The CPU 1513F-1 PN doubles the work memory. It handles moderate safety programs with multiple safety zones, safe speed monitoring, and more complex safety interlocking.
Best for: Standard OEM machines with moderate I/O and safety requirements. The most popular F-CPU for machine builders.
CPU 1515F: for larger safety applications
The CPU 1515F-2 PN provides 750 KB of work memory. Choose this when your combined standard + safety program exceeds 400 KB.
Best for: Multi-station lines with extensive safety zones and safe motion on multiple axes.
CPU 1516F: when you need PROFIBUS + safety
The CPU 1516F-3 PN/DP adds a built-in PROFIBUS DP Master. If your plant has legacy PROFIBUS safety devices (PROFIsafe over PROFIBUS), this is the F-CPU you need.
Best for: Systems with both PROFINET and PROFIBUS safety devices, or brownfield upgrades where PROFIBUS safety infrastructure is already installed.
CPU 1517F: maximum safety performance
The CPU 1517F-3 PN/DP offers 3 MB of work memory and 10 ns bit performance. It is designed for large-scale safety applications with hundreds of safety I/O points, complex safe motion profiles, and extensive safety diagnostics.
Best for: Automotive assembly lines, large packaging plants, and applications with 50+ safety-related devices.
F-I/O modules: safety-rated inputs and outputs
An F-CPU alone does not make your system safe. You also need F-I/O modules — safety-rated input and output modules that comply with SIL 3 / PL e.
We carry the following S7-1500 F-DI module:
| Part Number | Function | Channels |
|---|---|---|
| 6ES7526-1BH00-0AB0 | F-DI (Fail-Safe Digital Input) | 16 channels, SIL 3 / PL e |
F-I/O modules perform internal plausibility checks on every channel. For dual-channel safety inputs (like an E-stop with two contacts), the module monitors both channels and detects discrepancies.
ET 200SP also offers F-I/O modules for distributed safety — allowing safety-rated inputs and outputs at remote stations connected to the F-CPU over PROFIsafe.
Safety programming in TIA Portal
Safety programming uses the same TIA Portal environment but with a dedicated Safety perspective:
- Add safety-certified function blocks. TIA Portal includes pre-certified safety blocks for common functions: E-stop monitoring (ESTOP1), safety door monitoring (SF_DOOR), two-hand control (SF_TWOHAND), muting (SF_MUTING), and others.
- Program in LAD or FBD only. Safety programs must use LAD (Ladder Diagram) or FBD (Function Block Diagram). SCL and STL are not permitted in the safety program.
- Set the F-monitoring time. Each F-I/O module has a configurable monitoring time. If the CPU does not receive a valid PROFIsafe telegram within this time, the module's outputs go to their safe state.
- Generate the safety signature. After compiling the safety program, TIA Portal generates a unique CRC signature. This signature must be confirmed during commissioning as proof that the downloaded program matches the validated version.
- Test and validate. Follow IEC 62061 or ISO 13849 validation procedures. Document every safety function, test every E-stop, and record the results.
TF-CPU: technology + fail-safe combined
If your application needs both safety AND advanced motion control, Siemens offers TF-CPUs that combine both capabilities:
| Part Number | Model | Capabilities |
|---|---|---|
| 6ES7511-1UK01-0AB0 | CPU 1511TF-1 PN | Motion + Safety, entry-level |
| 6ES7515-2UM01-0AB0 | CPU 1515TF-2 PN | Motion + Safety, mid-range |
| 6ES7516-3UN00-0AB0 | CPU 1516TF-3 PN/DP | Motion + Safety + PROFIBUS |
| 6ES7517-3UP00-0AB0 | CPU 1517TF-3 PN/DP | Motion + Safety + PROFIBUS, high-end |
TF-CPUs handle standard automation, safety, and motion control — all in one CPU. This is the architecture Siemens recommends for modern machine building where safe motion (SLS, SLP, SOS, SS1, SS2, SDI) is required alongside servo or stepper control.
Safe motion functions
S7-1500 F-CPUs and TF-CPUs support these safe motion functions when paired with safety-certified Siemens drives (SINAMICS S120, G120, etc.):
| Function | Abbreviation | What It Does |
|---|---|---|
| Safe Torque Off | STO | Removes torque from the motor (most basic safety function) |
| Safe Stop 1 | SS1 | Controlled deceleration followed by STO |
| Safe Stop 2 | SS2 | Controlled deceleration followed by standstill monitoring |
| Safe Operating Stop | SOS | Monitors that the motor stays at standstill |
| Safely Limited Speed | SLS | Monitors that the motor does not exceed a speed limit |
| Safe Limited Position | SLP | Monitors that the motor stays within a position range |
| Safe Direction | SDI | Monitors that the motor only moves in one direction |
All safe motion functions communicate between the F-CPU and the drive over PROFIsafe (via PROFINET or PROFIBUS).
Frequently asked questions
Can I use a standard (non-F) CPU for safety applications?
No. Only F-CPUs are certified for SIL 3 / PL e safety functions. A standard CPU can work alongside external safety relays for basic E-stop circuits, but it cannot execute safety-certified logic internally. If your machine directive or risk assessment requires a safety-rated controller, you must use an F-CPU.
Can I run standard (non-safety) programs on an F-CPU?
Yes. An F-CPU runs standard programs exactly like a standard CPU. The safety program runs in addition to — not instead of — the standard program. If you buy an F-CPU but do not write a safety program, it functions identically to the equivalent standard CPU (just with more work memory). Many engineers choose F-CPUs even when safety is not needed today, so the hardware is ready if safety requirements are added later.
Do I need special TIA Portal licenses for safety programming?
Yes. Safety programming requires the STEP 7 Safety option license (an add-on to STEP 7 Professional). This license provides the Safety Administration editor, pre-certified safety function blocks, and the safety signature generation tool. Without this license, you can configure the F-CPU hardware but cannot create or edit the safety program.
What happens if the F-CPU detects an internal error?
If the F-CPU detects any inconsistency in the redundant safety calculation, it forces all F-I/O outputs to their safe state (defined during configuration, typically all outputs OFF). The standard program may continue running if the error is limited to the safety partition. The CPU logs the event in the diagnostics buffer with a detailed error code. The safe state persists until the fault is cleared and the safety program is re-acknowledged.
Build your safety system
We stock the full range of S7-1500 F-CPUs and F-I/O modules. For a complete overview of the S7-1500 platform, see our S7-1500 Buyer's Guide. Browse our Siemens PLC Systems collection or view all Siemens products. Request a quote with your safety requirements for a tailored recommendation.