As one of the first industrial control manufacturers to enter the Chinese market, Siemens has maintained a strong and dominant position in the industry for many years. Its PLCs are well-known for their high quality, stable networking capabilities, and open architecture, which make them popular among industrial control professionals. The programming software is also well-organized and user-friendly, contributing to its widespread use in domestic industrial automation systems.
While many people have some familiarity with Siemens PLCs, not everyone is fully aware of the various communication methods available. In this article, we will explore some of the most common communication protocols used by Siemens PLCs.
First, PPI Communication
PPI (Point-to-Point Interface) is the basic communication method used by S7-200 CPUs. It can be implemented through the built-in ports (PORT0 or PORT1), making it the default communication protocol for these devices.
Second, RS485 Serial Communication
This is a widely supported communication method, especially for third-party devices. Siemens S7 PLCs can use the free port mode to communicate via serial connections. In simple cases, only the XMT (transmit) instruction is needed to send data to devices like printers or inverters. However, all communication must be programmed within the S7 PLC. When using the free port mode, users can control the communication port using instructions such as XMT, RCV, and interrupts for both sending and receiving data.
Third, MPI Communication
MPI (Multi-Point Interface) is a relatively straightforward communication method. It supports speeds ranging from 19.2 Kbit/s to 12 Mbit/s, and can connect up to 32 nodes over a maximum distance of 50 meters. Repeaters can extend this distance, although they also consume a node. Common devices connected via MPI include S7-200 PLCs, HMI panels, programming tools, ET200S modules, and RS485 repeaters. There are three main ways to communicate between Siemens PLCs using MPI: global data packets, non-configured connections, and configured connections.
Fourth, Ethernet Communication
Ethernet was first introduced in 1968 at the University of Utah. In 1972, Metcalfe and David Boggs developed a network connecting multiple ALTO computers with an EARS laser printer, marking the world's first personal computer LAN. This network was named "Ethernet" after the concept of electromagnetic waves traveling through a medium called "ether." In 1979, DEC, Intel, and Xerox standardized the technology. Over time, Ethernet evolved to support various media, including thin coaxial cables, twisted pairs, CATV coaxial cables, and fiber optics. Today, Ethernet is one of the most widely used network technologies due to its high speed, rich resources, and ease of use and maintenance.
Fifth, PROFIBUS-DP Communication
PROFIBUS-DP is an open fieldbus system that adheres to European and international standards. It offers a compact structure and high-speed, stable transmission, making it ideal for communication between PLCs and distributed I/O devices on the factory floor.
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