How to wire the Siemens S7-1200 PLC

The wiring of the Siemens S7-1200 PLC needs to be designed according to the specific module configuration and application scenarios. The following are the basic wiring steps and precautions:

 I. Power Supply Wiring

1. **Power Supply Types**

   - **AC/DC Power Module** (such as PS 1207):

     - Connect L/N to the 220V AC power supply, and output +24V/0V to the PLC main unit and expansion modules.

   - **DC Power Module** (such as PS 1202):

     - Connect to the 24V DC power supply and ensure the correct polarity.

2. **Grounding**

   - The grounding terminals (PE) of all modules need to be reliably connected to the protective ground to avoid interference.

 II. Input/Output (I/O) Wiring

 1. Digital Input (DI)

- **Dry Contact Input** (such as buttons, proximity switches):

  - Connect one end to the DI terminal of the PLC (such as I0.0), and the other end to 0V.

- **Wet Contact Input** (such as NPN/PNP sensors):

  - For NPN type: Connect the sensor output terminal to the DI terminal, connect the positive pole of the power supply to the sensor +, and the negative pole to 0V.

  - For PNP type: Connect the sensor output terminal to 0V, and connect the positive pole of the power supply to the DI terminal and the sensor +.

 2. Digital Output (DO)

- **Relay Output** (such as Q0.0):

  - Connect one end of the load to the DO terminal, and the other end to the positive pole of the power supply (which needs to match the load voltage).

- **Transistor Output** (such as Q0.0):

  - Connect one end of the load to the positive pole of the power supply, and the other end to the DO terminal. An external freewheeling diode is required for protection.

 3. Analog Input (AI)

- **Voltage Signal** (such as 0-10V):

  - Connect the positive signal to AI+, the negative signal to AI-, and short-circuit AI- and COM.

- **Current Signal** (such as 4-20mA):

  - Connect the positive signal to AI+, the negative signal to AI-, and leave the COM terminal floating.

# 4. Analog Output (AO)

- **Voltage Output** (such as 0-10V):

  - Connect AO+ to the positive pole of the load, and AO- to the negative pole of the load.

- **Current Output** (such as 4-20mA):

  - Connect AO+ to the positive pole of the load, AO- to the negative pole of the load, and a 250Ω resistor needs to be connected in series to convert it into a voltage.

 III. Communication Interface Wiring

1. **RS485/Modbus RTU**

   - Use shielded twisted-pair wires. Connect A+ to the TX/RX+ of the PLC, B- to TX/RX-, and connect the terminal resistor (120Ω) as required.

2. **PROFINET**

   - Use an RJ45 network cable to connect to the PROFINET interface of the PLC, supporting daisy chain or star topology.

# IV. Expansion Module Wiring

- **Signal Module (SM)**: Connect to the bus connector on the back of the PLC main unit through a U-shaped connector.

- **Communication Module (CM)**: Insert directly into the expansion slot on the side of the PLC main unit.

 V. Precautions

1. **Power-off Operation**: Disconnect all power supplies before wiring to avoid short circuits or module damage.

2. **Voltage Matching**: Confirm the voltage level of the input/output modules (such as 24V DC or 230V AC).

3. **Anti-interference**: Analog signal wires need to be shielded and kept away from power lines, and twisted-pair wires can be used for digital signal wires.

4. **Load Capacity**: The maximum current of the relay output is usually 2A per point, and that of the transistor output is 0.5A per point.

 VI. Example: Connecting a Temperature Control Switch (Taking Digital Input as an Example)

1. **Output Types of the Temperature Control Switch**:

   - Normally Open (NO): When the temperature control switch operates, it closes and is connected to the DI terminal and 0V.

   - Normally Closed (NC): When the temperature control switch operates, it opens and is connected to the DI terminal and 0V.

2. **PLC Program Logic**:

   - Trigger the corresponding control logic (such as starting/stopping the heater) by reading the status of the DI signal.

According to the specific module models (such as CPU 1214C, SM 1221, SM 1231, etc.) and application requirements, the wiring method may need to be adjusted. It is recommended to refer to the official Siemens manual or the hardware configuration instructions in the TIA Portal software.