S7-1200 vs S7-1500: PLC Selection Guide

1. Why the decision matters

A controller selected for a machine often stays in service for ten to fifteen years. Changing families later may require reworking the program, relinking HMI tags, carrying duplicate spares and retraining technicians.

The CPU purchase price is only one part of total cost of ownership (TCO). Engineering hours, commissioning, maintenance and future expansion all contribute over the machine lifecycle.

2. Positioning the S7-1200 and S7-1500 families

The S7-1200 is a compact family. Some CPUs include digital and analog I/O, with expansion through signal boards, signal modules and communication modules. It is often suited to standalone machines and compact applications where footprint and hardware cost matter.

The S7-1500 is modular, with I/O installed as separate modules and resources for more demanding applications. It is often used on larger lines, coordinated motion applications and systems requiring detailed diagnostics or broader plant integration.

3. Performance and memory

The S7-1500 generally offers higher execution performance and more memory for applications combining machine sequences, data handling and communication. An S7-1200 may be sufficient for a standalone machine with straightforward sequencing; verify the sizing for the exact CPU.

Estimate the program, data blocks and recipe storage, then compare the requirement with the current data sheet. Measure cycle time in TIA Portal during commissioning and leave headroom for future changes.

4. Communication and connectivity

Both families support PROFINET device communication and can connect to HMI, SCADA and higher-level systems. Differences depend on the module combination, connection count and project functions.

Motion control using PROFINET IRT and isochronous operation is generally associated with S7-1500 applications. OPC UA functions and supported network topologies depend on the CPU, modules and firmware. Check current documentation for the exact order number.

5. Diagnostics and maintenance

The S7-1500 provides more detailed diagnostics and Trace functions that can record signal behaviour over time. These tools can help shorten investigations into intermittent faults and commissioning issues.

The S7-1200 offers diagnostics for many compact machines. Before selecting a family, check that its diagnostic workflow fits the experience and tools available to the maintenance team.

6. Compare total cost of ownership

Compare the complete configuration and lifecycle cost, not just the CPU price:

CPU, I/O, communication modules and power supply for the actual configuration;
engineering, testing, documentation and commissioning hours;
the number of order numbers required for spare parts;
technician training and the HMI licensing model;
likely machine or line expansions over the coming years.

7. Decision table: what the project requirements suggest

Criterion	Points toward S7-1200	Points toward S7-1500
Application size	Standalone machine, low or moderate I/O count	Multi-station line or high I/O count
Motion	Simple drives, no coordinated axes	Several coordinated servo axes
Diagnostics	Standard diagnostics meet the need	Trace and detailed diagnostics are required
Communication	Standard PROFINET and few connections	More network segments and wider integration; verify required functions
Panel space	Compact configuration	Space is available for a modular rack
Hardware budget	Strict CPU and I/O budget	The project justifies the added resources
Standardisation	Standalone machine with no required common spares	Common family and library across the plant

8. Three automation scenarios

A. Standalone labelling machine

A machine with about 40 digital inputs, 24 digital outputs, four analog signals, one HMI and no coordinated motion. An S7-1200 may meet the footprint and budget requirements if memory, communication and expansion have been checked. Document the expansion limit.

B. Four-station packaging line

A line with servo axes, traceability data sent to an MES, several PROFINET segments and frequent commissioning changes. An S7-1500 may be a suitable choice; standardisation can make function blocks reusable between stations and lines.

C. Hybrid architecture

The main line uses an S7-1500, while an independent auxiliary machine uses an S7-1200 and communicates over PROFINET. Define the data exchange clearly and give maintenance a consistent diagnostic view of both controllers.

9. Common mistakes

Choosing by price alone, then discovering memory or communication limits after the machine is built.
Leaving no headroom for I/O or program expansion.
Treating remote I/O as a minor detail; the station has its own cost and diagnostic requirements.
Checking functional safety requirements only after the program has been written.
Underestimating commissioning time when diagnostics do not match the faults the team must find.

10. Standardisation as a plant strategy

Using one controller family on most machines can reduce spare-part variety, training effort and engineering work. Set a default family for new machines and allow exceptions when a documented requirement justifies them. Keep the decision and its assumptions in the project file.

A practical exercise for your project

Count digital inputs and outputs, analog signals and special I/O.
List each communication link: HMI, SCADA, other controllers, drives, MES or ERP.
State whether coordinated motion is required and how many axes are involved.
Estimate programming effort and required memory, leaving headroom for changes.
Complete the decision table and justify the recommended family in one paragraph.
Identify the requirement that, if changed, would alter the recommendation.

Checklist before selecting a CPU

Read the current data sheets for the exact order numbers under consideration.
Check the TIA Portal version and firmware notes required for each CPU.
Confirm functional safety requirements with the safety engineer before fixing the CPU.
Keep a written selection rationale in the project folder.

Siemens PLC terms and technologies used in projects

These terms commonly arise when selecting, programming or modernising a Siemens PLC. Exact compatibility depends on the CPU order number, firmware, licences and TIA Portal version.

SIMATIC S7-1200: A family of compact PLCs for machine automation and small to medium applications. Select the CPU based on signal count and type, communications and required functions.
SIMATIC S7-1500: A modular PLC family used where processing, diagnostics, communications or motion-control requirements are broader. Check the exact CPU and module configuration.
SIMATIC S7-300 migration: Existing S7-300 installations may need a migration assessment. Inventory hardware order numbers, STEP 7 software, PROFIBUS/PROFINET networks, HMI equipment and acceptable downtime before choosing a replacement platform.
TIA Portal and STEP 7: Siemens engineering tools for hardware configuration, PLC programming, diagnostics and maintenance. Check project versions and compatibility with the target CPU and firmware.
WinCC, HMI and SCADA: WinCC provides visualisation and operator-interface solutions. For a new project or migration, check the Runtime version, tag count, recipes, alarms and connections between HMI, PLC and SCADA.
PROFINET and PROFINET IRT: Industrial Ethernet technologies used for communication between PLCs, remote I/O, HMI and drives. Verify real-time requirements, topology and supported devices for the intended architecture.
PROFIBUS DP: A fieldbus found in existing plants. For a modernisation project, document stations, addresses, GSD files and the migration or integration strategy.
OPC UA and MES/ERP integration: OPC UA can support data exchange between automation and higher-level systems. Verify CPU support, certificates, security, the information model and MES or ERP requirements.
SIMATIC ET 200 and distributed I/O: Remote I/O families place input and output modules near the machine. Sizing considers modules, network, update times and diagnostics.
Motion control, axes and drives: For positioning and coordinated motion, assess axis count, synchronisation functions, drive communication and the performance required by the machine cycle.
Fail-safe CPUs and functional safety: Safety requirements come from risk assessment and must be validated by competent personnel. CPU selection does not replace safety-function design and validation.

Frequently asked questions about Siemens PLCs

What is the difference between Siemens S7-1200 and S7-1500?

Compare them for the actual application: CPU performance, memory, I/O, communications, diagnostics, motion and total cost. There is no single best choice for every machine.

How should an S7-300 PLC migration be planned?

Start with a hardware and program inventory, then check remote I/O, PROFIBUS or PROFINET, HMI/WinCC, licences and shutdown windows. Test conversion and rollback plans before production changes.

What should be checked for TIA Portal, WinCC and PROFINET?

Confirm software and firmware versions, exact component compatibility, network topology and the functions the project depends on. Consult Siemens documentation for the selected order numbers.

Should a new machine use an S7-1200 or S7-1500?

Choose based on I/O, cycle time, communications, motion and safety needs, diagnostics, expansion and maintenance skills. Record the criteria and planned headroom.