Key Takeaway
Selecting the right S7-1500 CPU depends on your project’s complexity:
For small-to-medium automation tasks: CPU 1511 or CPU 1513
For mid-to-large tasks with higher communication/memory needs: CPU 1515 or 1517
For extremely high-performance needs (batch processes, motion control, redundancy): CPU 1518
Factors to consider include I/O requirements, number of motion axes, communication needs (Profinet/Profibus), safety demands (if using Failsafe CPUs), and memory size. Also, consider future scalability. Siemens provides helpful configuration tools like the TIA Selection Tool to guide appropriate CPU choice based on your system architecture, cycle time demands, and redundancy needs. Always overestimate slightly for long-term flexibility and upgrades.
Factors: Processing Speed and Memory Requirements
When selecting an S7-1500 CPU, the first thing you need to understand is your application’s processing needs. Ask yourself, how complex is the logic you’re going to write? Do you need to process large amounts of data in real time?
S7-1500 CPUs come in various performance classes. If your project involves high-speed operations, data logging, PID loops, or communication with multiple devices, go for a CPU with higher processing speed and larger working memory.
For example, a CPU 1518 has more power and memory than a CPU 1511. Don’t over-spec if you’re working on a small machine control application—a mid-range CPU like 1512 or 1513 might be ideal.
Memory size also affects your ability to log data, hold large logic blocks, or manage extensive user-defined data types (UDTs). It’s not just about speed—it’s about how much the CPU can handle.
A good practice? List your I/O count, expected data processing load, and future scale needs, then map them to CPU specs. That’s how you prevent both underperformance and overspending.
Built-In vs Extended Communication Support
Today, industrial communication is not an add-on. It’s a necessity. When evaluating the S7-1500 CPU, look at the onboard interfaces.
Most CPUs come with PROFINET as standard. But if your setup involves multiple networks like PROFIBUS, Modbus TCP, OPC UA, or industrial Ethernet variants, you need to verify whether the CPU supports them directly or requires additional modules.
Some higher-end CPUs offer integrated communication protocols like OPC UA or MQTT, which can reduce dependency on external communication processors. This not only saves rack space but also simplifies system architecture.
So, if your plant needs cloud connectivity or IT/OT integration, go for a CPU that includes open communication options. That way, you future-proof your setup and cut down integration time.
Communication is the backbone of modern automation. Select your CPU like your system depends on it—because it does.
Comparison Between Different S7-1500 CPU Models
Choosing between models like CPU 1511, 1512, 1513, and 1518 can feel like a maze. But it becomes easy once you break it down.
Each model comes with differences in processing speed, memory capacity, number of I/O devices supported, and communication features.
CPU 1511: Great for small machines, limited I/O
CPU 1512/1513: Perfect for mid-level applications, moderate I/O load
CPU 1515/1516: Higher performance, supports large applications with complex logic
CPU 1518: Industrial-grade powerhouse, suitable for high-end systems and data-intensive tasks
Look at your current project and anticipate growth. Are you planning to add more machines or lines in the future? Then a higher-end CPU makes sense even if it’s not fully utilized on Day 1.
Also, note the firmware compatibility with TIA Portal versions. Keeping everything aligned helps avoid upgrade headaches.
Your CPU is not just a controller. It’s your project’s brain. Choose it wisely.
Application Size and Expansion Planning
Don’t just think about today’s need. Think 3 years ahead.
A common mistake engineers make is selecting a CPU based only on current requirements. But what if your project expands? What if you add vision systems, more I/Os, or new machines?
Siemens S7-1500 CPUs vary in terms of the number of modules they support, both in terms of digital/analog I/Os and communication modules. Check how many rack expansions the CPU allows. Also, some CPUs are better suited for distributed setups using ET 200SP or PROFINET I/O devices.
Planning for expansion means fewer surprises. You won’t need to replace the CPU later, which could mean downtime and extra costs.
Always select a CPU that gives breathing room. Growth is good. Be ready for it.
Redundancy and High-Availability Considerations
High-availability systems aren’t just for power plants or big factories anymore. Even mid-sized automation lines need it, especially when downtime costs are high.
If your application requires redundancy, look for S7-1500R/H models. These offer CPU and communication redundancy. That means, if one CPU fails, the second one takes over automatically without data loss.
For critical systems—like pharma, chemical, or food processing—where stoppage equals serious loss, redundant CPUs are worth the investment.
Also, consider battery backup, firmware updates, and hot-swappable modules. They may sound advanced, but they become life-savers in real-world operations.
It’s not just about specs. It’s about peace of mind.
Conclusion
Selecting the right S7-1500 CPU isn’t just a technical decision. It’s a strategic one. The model you choose determines your system’s future flexibility, performance, and reliability.
Think beyond today. Evaluate your processing needs, communication protocols, future expansions, and how much downtime your application can tolerate.
When in doubt, go a level higher—not because it’s expensive, but because it’s scalable. Better to have extra capacity than fall short when your business grows. Let the S7-1500 series become your system’s backbone, not its bottleneck.