Key Takeaway
The S7-1200 system allows connection of:
Up to 3 communication modules (CM)
8 signal modules (SM) for digital/analog I/Os
1 signal board (SB) for small expansions
Exact limits depend on CPU type (e.g., CPU 1212C, 1214C, 1215C). Communication modules provide interfaces for RS232, RS485, Modbus RTU, GPRS, and more. Signal modules expand standard I/O beyond what is built into the CPU. Signal boards plug directly onto the CPU front panel and add compact expansion without increasing panel size. These modular options allow the S7-1200 to adapt from simple applications to moderately complex networked automation projects without replacing the CPU.
CPU Module Options and Variants
At the heart of every S7-1200 system is its CPU module. Think of it as the brain of your control system. Siemens offers several variants of S7-1200 CPUs like CPU 1211C, 1212C, 1214C, 1215C, and 1217C. Each one offers different memory sizes, processing speeds, and I/O capabilities.
New engineers often ask, “Which CPU should I use?” It depends on your application. For basic tasks, a 1211C may be sufficient. But for more complex automation, the 1215C or 1217C is the better fit. These CPUs come with built-in digital inputs/outputs, PROFINET communication, and integrated technology functions like high-speed counters and PID control.
What makes these CPUs versatile is that they can be expanded using other modules, which brings us to the next component in the system.
Signal Modules (Digital/Analog IO)
Need more inputs and outputs than your CPU offers? That’s where Signal Modules (SMs) come in. These modules are used to expand digital or analog I/O capacity.
For example, the SM1221 adds digital inputs, while the SM1222 adds digital outputs. If you need analog I/O, SM1231 (analog input) and SM1232 (analog output) are your go-to options.
Each SM is connected directly beside the CPU, maintaining a compact footprint. These modules are especially useful in field applications where multiple sensors or actuators are involved.
When planning your system, always consider the number of devices you’re connecting. Overloading the CPU’s internal IO limits performance and troubleshooting becomes harder. Signal Modules keep things scalable and organized.
Communication Modules (CM, CP)
In today’s factories, devices must talk to each other. That’s where Communication Modules (CM) and Communication Processors (CP) help.
The CM1241 module provides point-to-point serial communication (RS232/RS485), which is helpful for legacy systems. The CP1243 module allows secure remote access using VPN and firewall functions, making it ideal for Industry 4.0 setups and remote diagnostics.
If you are integrating barcode scanners, drives, or HMIs over Modbus or Profibus, these modules become essential. They offload communication tasks from the CPU and ensure seamless data flow.
As a beginner, focus on understanding what protocols your field devices support, and then match them with the right communication module. This is crucial for system compatibility.
Signal Boards and Add-On Boards
Want to add a few more IOs but don’t want a full expansion module? Signal Boards (SBs) are the answer. These compact boards plug directly onto the CPU and offer small additions like 4 DI or 2 AI channels.
They’re cost-effective, space-saving, and perfect for small upgrades. Similarly, Add-On Boards can be used for functions like RS485 communication or analog signal conditioning.
Use these when you have minor additional needs that don’t justify a full module. It keeps your panel compact and your wiring minimal.
Remember: Signal Boards don’t increase the system footprint. But only one board can be attached to each CPU, so choose wisely.
Mounting and Slot Limits for Module Expansion
Here’s where planning gets technical. The S7-1200 allows up to 8 modules (excluding Signal Boards) to be added alongside the CPU. That includes any mix of Signal Modules and Communication Modules.
Physically, these modules snap together on a DIN rail. Electrically, they draw power from the CPU’s internal bus, so be mindful of the power budget.
You should also watch out for total current draw and update time. The more modules you add, the longer your scan cycle might get. This can affect real-time control accuracy, especially in motion control applications.
Use TIA Portal to configure your hardware and simulate your setup. It warns you if you exceed the limits.
Conclusion
The Siemens S7-1200 supports a robust set of modules including CPU variants, Signal Modules, Communication Modules, Signal Boards, and Add-On Boards. Each plays a vital role in tailoring the PLC to match specific industrial needs.
Whether you’re automating a basic machine or building a complex control network, the modular design ensures you can scale up intelligently without replacing the core system. For new engineers, knowing what each module does, and how to select the right one, is key to building reliable automation systems.
Get hands-on, explore TIA Portal, and start with a basic setup. The S7-1200 will grow with you as your engineering skills expand.