Use this Siemens PLC selection guide to choose the right SIMATIC PLC for your application, including S7-1200, S7-1500, I/O, safety, motion, and networking.
Commercially focused selection guide explaining how to choose a Siemens PLC based on I/O requirements, CPU performance, memory, communications, motion, safety, environmental conditions, machine complexity, scalability, and lifecycle requirements.
Choosing a Siemens PLC is not simply a matter of comparing CPU speed, memory size, or the number of I/O points. The correct SIMATIC controller must match the machine architecture, control complexity, communication requirements, safety functions, motion demands, environmental conditions, expansion plans, and lifecycle expectations of the application. A controller that looks inexpensive on the purchase order can become an expensive engineering mistake if it lacks the performance, network architecture, diagnostics, or expansion capability the plant will need later.
In short, choose the Siemens PLC around the application—not around the CPU price. For compact machine automation, the SIMATIC S7-1200 family is generally the starting point, while the S7-1500 is intended for more demanding automation, larger systems, higher performance, and more sophisticated control architectures. For distributed I/O, safety, harsh environments, and decentralized machine designs, the controller must also be evaluated together with the appropriate SIMATIC ET 200 architecture rather than as an isolated PLC.
What Is a Siemens SIMATIC PLC?
A programmable logic controller, or PLC, is the real-time control engine of an industrial automation system. It reads inputs from sensors and field devices, executes the control program, makes decisions, and commands outputs such as motors, valves, contactors, actuators, drives, and other equipment.
The Siemens SIMATIC platform extends considerably beyond the PLC CPU itself. A modern system can include the controller, distributed I/O, HMI, drives, safety devices, industrial networking, engineering software, diagnostics, and higher-level SCADA or manufacturing systems.
This distinction matters because selecting a PLC by CPU specifications alone is a common engineering mistake. A controller may have sufficient processing power but still be the wrong choice because it does not provide the required communication architecture, motion functionality, safety capability, I/O density, or environmental rating.
Automation Pioneer’s Siemens PLC Systems portfolio reflects the breadth of the Siemens ecosystem, including S7-1200, S7-1500 and legacy SIMATIC platforms. (Automation Pioneer Siemens PLC Systems)
The First Decision: S7-1200 or S7-1500?
For most new Siemens PLC projects, the first major decision is whether the application belongs in the SIMATIC S7-1200 family or the SIMATIC S7-1500 family.
Siemens currently positions the S7-1200 G2 as a Basic Controller for compact and cost-conscious automation applications. It combines a compact form factor with enhanced performance, communications, scalability, safety options, and integrated motion capabilities.
The S7-1500 sits at the Advanced Controller level. It is designed for higher-performance automation, more complex processes and machines, sophisticated communications, and demanding control requirements. Siemens also integrates S7-1500 with TIA Portal for configuration, programming, diagnostics, and maintenance.
When the S7-1200 Makes Sense
Choose an S7-1200 when the application is relatively compact, the I/O count is manageable, and the control program does not demand the processing capacity or architecture of a larger controller.
Typical applications include standalone machines, conveyors, packaging equipment, pumps, compressors, material handling equipment, small production cells, HVAC systems, utility skids, and moderately complex process equipment.
The S7-1200 is particularly attractive when physical cabinet space and project cost matter. Its modular expansion approach also means you do not have to buy a large controller simply because the application may need a modest amount of expansion later.
The newer S7-1200 G2 deserves particular attention for new machine designs because Siemens has expanded the platform’s performance, communications, safety, and motion capabilities. Siemens states that the G2 supports integrated motion functionality and is intended for compact machine concepts and cost-efficient automation.
When the S7-1500 Is the Better Choice
Move to the S7-1500 when the application becomes processor-intensive, highly networked, safety-critical, motion-intensive, or significantly larger in scale.
An S7-1500 is usually the stronger choice when the machine or plant has substantial distributed I/O, demanding sequence execution, advanced diagnostics, extensive HMI/SCADA integration, sophisticated motion control, or future expansion requirements.
The S7-1500 also provides a much broader range of CPU performance classes and specialized controller variants. Siemens identifies standard, fail-safe, technology, redundant/high-availability, and software-controller options within its broader controller architecture.
The practical rule is straightforward: if you already know the application is pushing the limits of an S7-1200, do not force it into the project simply to reduce the initial hardware bill.
Start With the Application, Not the PLC Catalog
Before selecting a CPU, write down what the control system actually has to accomplish.
You need to know the number and type of field signals, required scan performance, communication networks, control loops, motion axes, safety functions, HMI requirements, SCADA connectivity, environmental conditions, and expected future expansion.
For example, a machine with 40 digital inputs, 30 digital outputs, several analog channels, two servo axes, a PROFINET network, an HMI, and safety interlocking has a fundamentally different controller requirement from a pump skid with eight digital inputs and six outputs.
The PLC should therefore be selected from the control architecture outward, not from the CPU price inward.
Automation Pioneer’s broader industrial automation and process control categories are useful when considering the PLC alongside drives, sensors, transmitters, HMIs, power supplies, relays, and other components that ultimately determine the automation architecture. (Automation Pioneer Product Categories)
Step 1: Calculate the I/O Requirement Properly
I/O capacity is one of the easiest specifications to misunderstand.
Do not simply count today’s sensors and actuators. Calculate the present requirement, identify the signal types, and then provide reasonable expansion capacity.
Digital inputs may include proximity switches, photoelectric sensors, pushbuttons, pressure switches, limit switches, and status contacts. Digital outputs may operate solenoid valves, contactors, stack lights, relays, and discrete actuators.
Analog signals require more careful consideration. A process may use 4–20 mA pressure transmitters, temperature transmitters, flow meters, load cells, RTDs, thermocouples, or voltage-based signals. These require appropriate analog modules rather than generic digital I/O.
You should also identify high-speed counting, pulse inputs, encoder signals, technology modules, and specialized measurement requirements before selecting the CPU.
Leave Expansion Capacity
A PLC design with exactly enough I/O for today’s machine is poorly engineered.
A better design normally reserves capacity for future sensors, additional actuators, production upgrades, spare channels, and design changes. The precise allowance depends on the application, but the principle is universal: do not design the control system with zero breathing room.
Distributed I/O can further change the calculation. Siemens’ ET 200 family provides centralized and decentralized I/O architectures, including compact cabinet-mounted systems and rugged systems designed for installation closer to the machine.
Step 2: Determine Whether You Need Centralized or Distributed I/O
The physical layout of the machine can be just as important as CPU performance.
If sensors and actuators are concentrated around one control cabinet, centralized I/O can be simple and economical. However, when field devices are distributed across a production line, large machine, building, or process plant, running individual signal cables back to one cabinet can create unnecessary wiring, terminal blocks, cabinet space, labor, and troubleshooting complexity.
Distributed I/O solves this by locating I/O stations closer to the equipment.
The SIMATIC ET 200 portfolio includes systems for cabinet installation as well as rugged field-mounted configurations. ET 200SP, ET 200MP, ET 200AL, ET 200pro and other variants target different installation and performance requirements.
For harsh environments, the choice becomes even more important. Siemens offers ET 200 systems designed for demanding environments, including solutions with enhanced ingress protection and specialized versions for outdoor, hygienic, or hazardous applications.
PROFINET Architecture Matters
Once distributed I/O enters the design, the network becomes part of the control system.
PROFINET is central to current Siemens automation architectures, providing communication between controllers, I/O, drives, HMI systems, and other industrial devices. S7-1500 CPUs have integrated PROFINET interfaces, while additional communication modules can extend network and communication capabilities.
Therefore, do not select a CPU first and figure out the network afterward. Design the network architecture at the same time.
Step 3: Evaluate Processing Performance and Scan Time
PLC performance is not simply about how fast a CPU executes one instruction.
The real question is whether the controller can consistently execute the complete application within the required process cycle.
A PLC program may contain sequencing logic, PID loops, motion calculations, communication tasks, diagnostics, recipe management, data logging, HMI communications, and safety processing. As these functions accumulate, controller loading increases.
For a basic machine, an S7-1200 can provide more than enough performance. For a large production line with extensive communications and motion, an S7-1500 provides a more appropriate performance envelope.
Do not select the smallest CPU that technically runs the program. Select a CPU that leaves reasonable performance headroom.
This is particularly important for machines that will evolve after commissioning. Additional recipes, diagnostics, production reporting, communication nodes, or motion functions can increase CPU loading significantly over the operating life.
Step 4: Choose the Right Communication Architecture
Industrial communication should be treated as a core design requirement, not an accessory.
Ask exactly what the PLC needs to communicate with:
HMI panels
SCADA systems
distributed I/O
variable frequency drives
servo drives
safety devices
barcode scanners
vision systems
robots
weighing systems
historians
MES platforms
plant networks
third-party PLCs
cloud or edge systems
The S7-1500 supports modern industrial communications including PROFINET, Industrial Ethernet, and OPC UA.
The S7-1200 platform also provides substantial communications capability, including Ethernet-based communication and OPC UA functionality on supported configurations.
Do Not Ignore Legacy Networks
Many existing plants still contain PROFIBUS, MPI, or older SIMATIC systems.
That changes the selection strategy.
If you are replacing an existing controller rather than building a greenfield system, compatibility with existing drives, remote I/O, HMIs, communication processors, and field devices can be more important than selecting the newest CPU.
In other words, brownfield automation projects require a migration strategy, not just a product selection.
Step 5: Decide Whether You Need Motion Control
Motion changes the PLC selection considerably.
If the application involves servo motors, synchronized axes, positioning, electronic gearing, camming, coordinated motion, or high-speed control, you should evaluate the PLC together with the drive architecture.
Siemens integrates motion functionality into its SIMATIC ecosystem and provides technology-oriented controller options. The S7-1200 G2, for example, includes integrated motion capabilities for applications ranging from axis positioning to more advanced coordinated movement.
The S7-1500 platform provides a broader performance range for demanding motion applications, including specialized technology CPUs.
The key is to define the motion requirement precisely. A single conveyor speed command is not equivalent to synchronized multi-axis motion. The PLC must match the actual control problem.
Step 6: Determine Whether Functional Safety Is Required
Safety should never be added as an afterthought.
If the machine includes emergency stops, safety gates, light curtains, two-hand controls, safe torque off, safe motion, safety scanners, or other safety functions, determine the required safety architecture before choosing the PLC.
Siemens provides fail-safe SIMATIC controllers and distributed fail-safe I/O. Depending on the architecture, safety functions can be integrated into the controller and communicated through PROFIsafe over PROFINET. Siemens documentation identifies fail-safe S7-1500 CPUs capable of combining standard and safety programs and supporting applications up to SIL 3 and PL e under applicable standards and conditions.
The correct approach is to start with the required Performance Level or Safety Integrity Level and then select the appropriate safety architecture.
Do not assume that a standard PLC becomes a safety controller merely because safety logic can be written into the standard program.
Step 7: Consider Redundancy and High Availability
For a small machine, a PLC failure may stop one production cell.
For a continuous process, power-generation facility, water-treatment plant, or critical production operation, a controller failure can create a much larger operational and financial impact.
That is where redundancy becomes relevant.
Siemens offers S7-1500 R/H CPUs for redundant and high-availability applications. These systems use synchronized controller architectures so a backup CPU can take over if the primary controller fails. Siemens also supports redundancy at different levels of the architecture, including controller, network, and interface components. If downtime has a very high cost, the question should not be “Can we afford two PLCs?”
The better question is “What does one uncontrolled production stoppage cost us?”
Step 8: Select the Engineering Environment Carefully
For modern SIMATIC S7 systems, TIA Portal is a major part of the engineering decision.
STEP 7 within TIA Portal is Siemens’ integrated environment for configuring, programming, and maintaining automation systems. It supports PLCs, distributed I/O, motion control, and safety within a common engineering platform.
This matters because engineering productivity directly affects the total cost of an automation project.
A controller that costs less but requires significantly more engineering effort can be more expensive over the life of the project.
TIA Portal also provides integrated diagnostics, device configuration, programming, and system-level engineering. This unified approach can reduce the amount of disconnected configuration work required across the automation system.
Think Beyond Initial Programming
The engineering environment also affects maintenance.
When a technician needs to troubleshoot a failed sensor, communication module, drive, or I/O station at 2 a.m., good diagnostics can be worth considerably more than a small difference in PLC purchase price.
For that reason, diagnostics, online monitoring, trace functions, hardware identification, fault reporting, and maintainability should be part of the PLC selection process.
Step 9: Match the PLC to the HMI and SCADA Strategy
The PLC is rarely operating alone.
Most modern industrial systems include an HMI, and larger installations may include SCADA, historian, MES, or other plant-level systems.
Siemens’ S7 controllers integrate into the broader SIMATIC environment, including HMI and WinCC technologies. Siemens’ training material specifically covers S7-1200 with WinCC Basic and S7-1500 with WinCC Advanced within TIA Portal.
Therefore, define the operator interface requirements before finalizing the controller.
A small standalone machine may need only a compact HMI. A production line may require centralized visualization, alarm management, recipe handling, trends, user management, historical data, and integration with higher-level systems.
The PLC must have enough communication capability and data-handling capacity to support the architecture without becoming overloaded.
Step 10: Consider the Physical Environment
The control cabinet may be clean and temperature-controlled—or it may be located beside a washdown system, furnace, compressor, chemical process, outdoor skid, or high-vibration machine.
That distinction matters.
Temperature, humidity, condensation, vibration, electromagnetic interference, dust, chemical exposure, water ingress, and hazardous-area requirements can influence the controller and I/O selection.
In harsh environments, the correct solution may be to move I/O closer to the machine using an appropriately rated ET 200 system rather than putting every component inside a centralized cabinet.
Siemens’ ET 200 portfolio includes systems designed for cabinet-free and harsh-environment applications, including IP65/67 and higher-protection solutions depending on the product family.
Step 11: Think About Lifecycle and Spare Parts
A PLC is not a disposable component.
Industrial control systems can operate for 10, 15, 20 years or longer. Consequently, the selection process should include lifecycle planning.
Ask whether the selected CPU will remain supportable, whether spare CPUs and I/O modules can be obtained, whether engineering software will remain available, and how future replacement will be handled.
This is particularly important for legacy SIMATIC S7-200, S7-300, and S7-400 installations. Automation Pioneer continues to list components from these older families, which can be useful for maintenance and retrofit projects. (Automation Pioneer Siemens PLC Systems)
However, maintaining an existing legacy system and specifying a legacy platform for a new greenfield project are two different decisions.
For a new installation, evaluate the current Siemens controller architecture first. For an existing installation, evaluate migration cost, compatibility, production downtime, engineering effort, and spare-part availability before deciding whether replacement or continued maintenance makes more sense.
Step 12: Calculate Total Cost of Ownership
The cheapest PLC is not necessarily the lowest-cost automation solution.
Total cost includes the CPU, I/O, communication modules, power supplies, network equipment, engineering software, HMI, programming labor, commissioning time, spare parts, maintenance, training, downtime, and future expansion.
Suppose an undersized controller saves $1,000 in initial hardware cost but requires additional communication modules, more complicated programming, premature replacement, and several days of engineering during an expansion.
That was not a $1,000 saving.
It was a deferred engineering cost.
Conversely, oversizing every machine with an S7-1500 simply because it is more powerful is also poor engineering. The objective is not maximum PLC capacity. The objective is the correct technical capacity with appropriate headroom at the lowest sensible lifecycle cost.
Siemens PLC Selection Decision Matrix
A practical selection process can be summarized as follows.
Choose S7-1200 G2 when the application is compact, cost-sensitive, machine-oriented, and requires standard control with moderate I/O, communications, and motion functionality.
Choose S7-1500 when the application requires higher processing performance, larger or more complex automation architecture, extensive communications, demanding motion, advanced diagnostics, or greater expansion capability.
Choose an S7-1500 F or appropriate fail-safe architecture when the safety assessment requires integrated programmable safety functions.
Choose S7-1500 R/H when controller redundancy and high availability justify the additional architecture.
Choose distributed ET 200 I/O when field devices are geographically distributed, cabinet wiring is excessive, machine-mounted I/O is advantageous, or environmental requirements demand a specialized remote I/O platform.
Evaluate legacy S7 platforms primarily in the context of existing installations, where compatibility, migration planning, and spare-part requirements may determine the most practical solution.
Common Siemens PLC Selection Mistakes to Avoid
Choosing the PLC Based Only on I/O Count
Two machines can have identical I/O counts and completely different controller requirements. Processing load, communications, motion, safety, and data requirements matter just as much.
Buying the Smallest CPU That Works Today
This approach creates unnecessary engineering constraints later. Provide sensible performance and memory headroom.
Ignoring the Network
If the system requires PROFINET, PROFIBUS, OPC UA, third-party Ethernet devices, drives, remote I/O, or SCADA, define those requirements before selecting the CPU.
Treating Safety as Ordinary Control
Safety functions require a properly engineered safety architecture. Do not attempt to retrofit functional safety after the controller has already been selected.
Forgetting the Environment
A controller suitable for a clean cabinet may not be suitable for direct machine installation, washdown, outdoor service, high vibration, or harsh industrial environments.
Ignoring Existing Equipment
A greenfield system can be designed around current technology. A brownfield system has to coexist with equipment already installed. That difference can completely change the best PLC choice.
A Practical Siemens PLC Selection Workflow
The most reliable way to select a SIMATIC PLC is to follow a structured engineering sequence.
First, document the machine or process requirements. Next, calculate digital, analog, high-speed, communication, safety, and technology I/O. Then determine centralized versus distributed architecture.
After that, define communication requirements and identify every device that must exchange data with the PLC. Determine whether the system requires motion, safety, redundancy, advanced diagnostics, or high availability.
Only then should you compare S7-1200 and S7-1500 CPUs.
Once the CPU family is selected, determine the specific CPU performance class, I/O modules, communication modules, power supply, network architecture, HMI, safety equipment, and engineering software.
Finally, validate the complete configuration against the application—not just the CPU datasheet.
Siemens itself provides a controller configurator that follows a similar selection philosophy, examining controller architecture, system performance, programming, motion control, and system architecture rather than treating the CPU as an isolated component.
Final Recommendation: Select the PLC as an Automation Platform
The right Siemens PLC is the one that fits the complete automation architecture, not simply the one with the lowest price or highest specification.
For many compact machine applications, the S7-1200 G2 is a logical starting point because it combines compact hardware with modern communications, safety, motion, and TIA Portal engineering. For larger, faster, more complex, or highly networked applications, the S7-1500 provides a substantially broader performance and architecture envelope. Distributed ET 200 I/O should then be considered wherever machine layout, wiring, environmental conditions, scalability, or decentralized control make centralized I/O impractical.
The best selection process therefore works backward from the application: define the control problem, define the architecture, define the performance requirements, and then select the PLC. That approach prevents both under-engineering and unnecessary overspending—and it produces an automation system that is easier to commission, troubleshoot, expand, and maintain.
For organizations sourcing Siemens automation hardware, Automation Pioneer provides access to a broad range of industrial automation components and PLC systems, supporting applications across manufacturing, process industries, energy, water, pharmaceuticals, food and beverage, and other industrial sectors. (Industries We Serve)
Frequently Asked Questions About Siemens PLC Selection
1. Which Siemens PLC is best for industrial automation?
There is no single Siemens PLC that is best for every application. The S7-1200 G2 is generally suited to compact and cost-sensitive automation, while the S7-1500 is better suited to advanced, high-performance, complex, and highly connected industrial applications.
2. What is the difference between Siemens S7-1200 and S7-1500?
The S7-1200 is positioned as a Basic Controller for compact automation, while the S7-1500 is an Advanced Controller designed for higher-performance and more complex applications. The S7-1500 provides a broader range of CPU performance and specialized architectures.
3. Is Siemens S7-1200 suitable for industrial machines?
Yes. The S7-1200 family is specifically designed for compact industrial automation and machine-control applications. The newer S7-1200 G2 adds enhanced performance, communications, safety, and motion capabilities.
4. When should I use an S7-1500 instead of an S7-1200?
Use an S7-1500 when the application requires significantly greater processing capacity, complex communications, advanced motion, extensive distributed I/O, sophisticated diagnostics, or high availability. It is also the more appropriate platform for many large machine and plant automation architectures.
5. Do Siemens S7-1200 and S7-1500 use TIA Portal?
Yes. Siemens integrates both controller families into TIA Portal, with STEP 7 providing the engineering environment for configuration, programming, diagnostics, and maintenance.
6. What Siemens PLC should I use for motion control?
The correct PLC depends on the number of axes, synchronization requirements, positioning accuracy, cycle requirements, drive technology, and application complexity. Both S7-1200 G2 and S7-1500 platforms provide motion-control capabilities, but demanding multi-axis applications may justify an S7-1500 technology-oriented solution.
7. What Siemens PLC should I use for safety applications?
Safety applications require an appropriately engineered fail-safe architecture. Siemens provides fail-safe S7 controllers and ET 200 safety I/O, with PROFIsafe used for safety communication in applicable architectures.
8. What is Siemens ET 200 used for?
SIMATIC ET 200 is a distributed I/O platform used to connect field sensors and actuators to controllers. It provides scalable centralized and decentralized I/O options for different cabinet, machine, environmental, safety, and process requirements.
9. Should I use a Siemens S7-300 or S7-400 for a new automation project?
For a new project, the current SIMATIC controller architecture should be evaluated first rather than automatically selecting an older platform. However, S7-300 and S7-400 equipment can remain relevant in existing plants where compatibility, migration cost, spare parts, and production downtime are important considerations. (Automation Pioneer Siemens PLC Systems)
10. How do I choose the correct Siemens PLC CPU?
Start with the application requirements: I/O count and type, processing load, communications, motion, safety, redundancy, environmental conditions, HMI/SCADA integration, expansion requirements, and lifecycle expectations. Then select the controller family and CPU performance class that provides sufficient capacity and practical headroom without unnecessarily increasing system cost.
Recommended Automation Pioneer Resources
To deepen your understanding of industrial automation, critical power infrastructure, and digital transformation, explore these related resources on Automation Pioneer:
- PLCs & Controllers
- Siemens Programmable Logic Controllers (PLCs)
- Siemens S7-1200 PLC
- Siemens S7-1500 PLC
- Siemens S7-1200 vs S7-1500
- Siemens TIA Portal
- Siemens PLC Programming
- Siemens PLC Communication
- Siemens PLC Troubleshooting
- Siemens PLC Accessories
- Siemens PLC Selection Guide
These articles offer additional technical insights into the automation, networking, monitoring, and control technologies that support modern industrial automation and mission-critical infrastructure.
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