Siemens programmable logic controllers (PLCs) are widely used in China and are commonly found in industries such as metallurgy, chemicals, and printing. The company offers a range of PLC models, including LOGO, S7-200, S7-1200, S7-300, and S7-400. Among these, the S7 series is particularly popular due to its compact size, fast processing speed, standardization, and network communication capabilities. It also provides strong functionality and high reliability. The S7 series can be categorized into micro PLCs (like S7-200), small-scale performance PLCs (like S7-300), and medium-to-high performance PLCs (like S7-400).
The working principle of Siemens PLCs involves three main stages: input sampling, user program execution, and output refresh. This entire process is referred to as a scan cycle, and the CPU repeatedly executes these steps at a set scanning speed during operation.
During the input sampling phase, the PLC reads all input states and data in a scan mode and stores them in the corresponding I/O map area. Once this phase is complete, the system moves on to the user program execution and output refresh stages. During these phases, the I/O map area remains unchanged even if the input states or data change. Therefore, for pulse signals, their width must be greater than one scan period to ensure they are captured correctly.
In the user program execution phase, the PLC scans the ladder diagram from top to bottom. When scanning each line, it first processes the control lines formed by the contacts on the left side, performing logical operations in a left-to-right, top-to-bottom order. Based on the results, it updates the status of the corresponding bit in the system RAM or the I/O map area. It may also execute specific instructions defined in the ladder diagram.
It’s important to note that during program execution, only the input points in the I/O map remain unchanged, while other outputs and internal devices may change. These changes affect subsequent ladder diagrams in the next scan cycle.
In the output refresh phase, after the user program is executed, the CPU updates all output latch circuits based on the data in the I/O map and drives the connected peripherals through the output circuit. This is when the actual output occurs.
The order of ladder diagrams significantly affects the outcome, and the results of the scan-based program execution differ from those of parallel relay logic. However, if the scan cycle time is negligible, the results are essentially the same.
For users new to the S7-200 PLC, getting started can be challenging. This guide introduces the initial steps for using the S7-200, helping you take your first steps in PLC programming. The goal here is to turn on an output when a switch is pressed.
To begin, you’ll need the following hardware and software:
- A computer with STEP 7 MicroWIN V4.0 SP6 programming software
- A PPI programming cable
- A Siemens S7-200 CPU (e.g., CPU224XP)
Once you have the necessary equipment, install the programming software properly. Make sure to use STEP 7 Micro/WIN, not STEP 7, which is for S7-300 PLCs. After installation, you should see the desktop icon for the software.
Next, connect the PLC. Ensure proper wiring, providing power to the PLC and connecting digital inputs appropriately.
To communicate with the PLC, connect the programming cable to the computer and the PLC's PORT0 or PORT1. Set the PLC mode switch to STOP. In the programming software, configure the PG/PC interface, select the PC/PPI cable, and set the communication interface to USB.
After successful configuration, you can test the connection. If everything is set up correctly, the software will detect the PLC.
Now, you can start writing the program. Open the programming interface, add normally open contacts and output coils, assign correct addresses, and download the program to the PLC.
Finally, debug the PLC by setting the mode switch to RUN and testing the input and output points. You can monitor the status of the inputs and outputs directly in the software to verify the program’s functionality.
This step-by-step guide helps beginners understand how to set up, program, and test a Siemens S7-200 PLC, making it easier to get started in industrial automation.
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