The programmable logic controllers (PLCs) manufactured by Siemens in Germany are widely used in China, particularly in industries such as metallurgy, chemicals, and printing. The Siemens PLC product line includes models like LOGO, S7-200, S7-1200, S7-300, and S7-400. The S7 series is known for its compact size, high speed, standardization, network communication capabilities, advanced features, and high reliability. These PLCs can be categorized into micro PLCs (such as the S7-200), small-scale performance PLCs (like the S7-300), and medium to high-performance PLCs (such as the S7-400).
**Siemens PLC Working Principle:**
When a PLC is put into operation, it typically goes through three main stages: input sampling, user program execution, and output refresh. Completing these three steps constitutes one scan cycle. Throughout the operation, the CPU of the PLC continuously performs these three steps at a specific scanning speed.
**Input Sampling:**
During the input sampling phase, the PLC reads all the input states and data in a scanning mode and stores them in the corresponding memory locations within the I/O map area. Once this phase is complete, the PLC moves on to the user program execution and output refresh stage. During these two phases, the status and data in the I/O map area remain unchanged even if the input conditions change. Therefore, if the input is a pulse signal, its width must be greater than one scan period to ensure it is captured correctly.
**User Program Execution:**
In the user program execution phase, the PLC scans the ladder diagram from top to bottom. When processing each rung, the control lines formed by the left-side contacts are scanned first, followed by logical operations in the order of left to right and top to bottom. Based on the results of these operations, the PLC updates the state of the relevant bit in the system RAM or the I/O map area, or executes any specified special function instructions.
It's important to note that during the execution of the user program, only the input points in the I/O map area remain unchanged. Other output points and internal devices may change their state or data. The results of the current scan will affect the ladder diagrams that use these coils or data, while the next scan will process the updated values.
**Output Refresh:**
After the user program execution is complete, the PLC enters the output refresh phase. During this time, the CPU updates all the output latch circuits based on the data stored in the I/O map area and drives the connected peripherals through the output circuit. This is when the actual output of the PLC takes place.
The order in which ladder diagrams are arranged can significantly affect the outcome. Additionally, the results of the scanning process differ from the parallel operation of hard-wired relay control systems. However, if the scan cycle is short enough relative to the overall operation, the difference becomes negligible.
**Siemens PLC Programming Tutorial:**
For many users new to the S7-200 PLC, several challenges can arise when getting started. This tutorial aims to guide you through your first steps in using the S7-200 PLC. The goal of this example is to turn on an output point when a switch is pressed. It covers the following key steps:
1. What hardware and software do you need?
2. How to wire the PLC?
3. How to communicate with the PLC?
4. How to write a program?
5. How to debug the PLC?
**Tools and Materials Required:**
- A computer running Windows XP SP2 with STEP 7 Micro/WIN V4.0 SP6 programming software.
- A PPI programming cable.
- A Siemens S7-200 CPU, such as the CPU224XP (part number: 6ES7 214-2BD23-0XB8).
**Step-by-Step Guide:**
**1. Hardware and Software Preparation:**
Ensure you have the correct hardware, including the S7-200 CPU and a PC/PPI cable. Install the STEP 7 Micro/WIN V4.0 SP6 software on your computer. Make sure you're not using the wrong version, as STEP 7 is designed for S7-300 PLCs and cannot program the S7-200.
**2. Wiring the PLC:**
Connect the PLC to a power source and provide a positive signal to the digital inputs. Ensure the wiring is secure and properly configured as shown in the wiring diagram.
**3. Communicating with the PLC:**
Connect the programming cable to both the computer and the PLC. Set the PLC’s mode switch to STOP. Then configure the PG/PC interface in the programming software, select the appropriate communication settings, and test the connection.
**4. Writing the Program:**
Open the programming interface and add a normally open contact and an output coil. Assign the correct addresses to the input and output points. Download the program to the PLC.
**5. Debugging the PLC:**
Set the mode switch to RUN and test the functionality by toggling the input switch. Use the status table feature in the software to monitor the input and output points in real-time and verify that the program is functioning as expected.
By following these steps, you’ll gain hands-on experience with the S7-200 PLC and be able to implement simple control functions effectively.
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