Key Takeaway
To power on the S7-1200 PLC, you must supply it with a stable 24V DC power source. Connect the positive wire to the designated 24V terminal and the negative wire to the 0V terminal on the CPU’s power supply connectors. Ensure correct polarity. Once powered, the CPU’s LEDs (such as RUN/STOP, ERROR) will illuminate, indicating operational status. If configured correctly, the CPU automatically executes the loaded user program when switched to RUN mode. Always verify wiring, input fuses, and grounding for safety before energizing the system. If external modules (I/O expansions or communication modules) are connected, they are powered through the CPU or separate supply if specified.
Power Supply Specifications and Requirements
Let’s start with the basics—the power supply. The Siemens S7-1200 CPU typically requires a 24V DC supply. This isn’t optional or adjustable—it’s essential. The acceptable range usually lies between 20.4V to 28.8V DC, but always check your specific CPU model’s datasheet to be sure.
The power consumption of the CPU module itself is quite low, around 2.5W to 3W. But if you’re using expansion modules or connecting sensors and actuators directly, calculate the total load and choose a power supply that can comfortably support it with a little headroom.
Make sure the power supply is industrial-grade and protected against overvoltage, short circuits, and surges. Mount it on a DIN rail close to the CPU for a neat and safe setup.
Wiring Connections for Powering the CPU
Now let’s talk connections. The S7-1200 CPU has dedicated terminals for power input—marked L+ (24V DC positive) and M (0V/GND). Always use properly insulated copper wires of appropriate gauge—typically 0.75 mm² or higher for DC power.
Here’s the trick: Keep your wiring clean and short. Run power wires separately from signal or analog cables to avoid interference.
Use ferrules at the wire ends for neat and reliable connections. Once wired, gently tug the wire to ensure it’s firmly seated in the terminal. No loose ends allowed!
LED Indicators and Boot Behavior
After powering it up, the first thing you’ll notice is the LED indicators. These tiny lights speak volumes about your CPU’s status.
You’ll see:
RUN/STOP LED: Blinks green during startup, steady green in RUN mode, and red for STOP mode.
ERROR LED: Off means all good. Red? That’s your cue to check diagnostics.
MC LED (Memory Card): Only relevant if you’re using a memory card—it’ll blink during read/write operations.
Boot behavior is usually quick. If the CPU was previously configured, it might boot into RUN or STOP based on last saved state. If it’s brand new or freshly reset, expect it to boot into STOP mode.
Visual Cue: Always watch the LEDs for quick diagnostics. They’re your first responders.
Startup Troubleshooting Steps
Nothing happens after switching on? Don’t panic. Let’s walk through a few checks.
Check the voltage at terminals L+ and M using a multimeter.
Inspect LED indicators. Are they off? It could mean no power or reverse polarity.
Review the wiring—look for loose wires or shorts.
Try a different power supply if the current one might be faulty.
Remove expansion modules temporarily to rule out excess power load.
Still no luck? Refer to the Siemens TIA Portal diagnostics or use the accessible diagnostic buffer.
Engineer’s Reminder: Never assume the CPU is faulty right away—80% of power-on issues are wiring or supply related.
Initial Configuration After Power-On
Once your CPU is powered and the LEDs show green, it’s time to prep the system for action.
Connect the PLC to your PC using an Ethernet cable.
Open TIA Portal and create a new project.
Detect and add your CPU model (S7-1200) to the project.
Assign the IP address if it hasn’t been done yet.
Download the project into the CPU and switch to RUN mode.
Always back up your configuration and use memory cards for critical projects.
Pro Setup Tip: Label your cables and document IP addresses. Makes life easier during future maintenance.
Conclusion
Powering on the S7-1200 is simple—but getting it right is critical. From selecting the proper power supply to checking wiring and understanding those LED indicators, every step plays a role in safe and reliable PLC startup.
For a new engineer, mastering these initial steps builds confidence and sets the foundation for more advanced automation work. And remember, when in doubt, check the wiring twice and always listen to what the LEDs are trying to tell you.
Stay safe. Stay sharp. And power up with confidence.