In modern industrial automation systems, the PLC (Programmable Logic Controller) is no longer an isolated island of information. The Siemens S7-1200 PLC is a widely used compact controller, and its core competitiveness lies not only in logical control but also in its powerful communication capabilities. Devices must interconnect through various components to execute commands, provide signal feedback, and exchange data, thereby achieving efficient collaborative work. Поэтому, a deep understanding of Siemens S7-1200 PLC basic communication knowledge is compulsory for every automation engineer. This guide goes beyond basic software operations (like TIA Portal) to explain the core elements of PLC communication from foundational concepts.
To understand communication, we must first clarify the objects of communication. PLC communication is generally divided into the following three categories:
Communication between PLCs: Used for data exchange and logical interlocking between multiple controllers.
Communication between PLC and Host Computer (HMI/SCADA): Realizing monitoring and data interaction between the PLC and HMI (Human Machine Interface), SCADA (Supervisory Control and Data Acquisition) systems, or industrial PCs.
Communication between PLC and Other Intelligent Devices: Например, communication with third-party devices such as variable frequency drives (VFD), servo drives, barcode scanners, and instrumentation.
To better master Siemens S7-1200 PLC communication skills, we need to understand the following key terms and distinctions:
These are the two most basic methods of data transmission.
Serial Communication: Data is transmitted bit by bit sequentially over a single transmission line.
Features: Communication control is relatively complex, but it requires fewer cables and has lower costs, making it suitable for long-distance transmission. Common examples include RS232, RS485 interfaces, and the ubiquitous USB interface.
Parallel Communication: Multiple bits of data are transmitted simultaneously over multiple parallel transmission lines.
Features: Fast transmission speed (multiple bits transmitted in one cycle), but it requires many cables. For long-distance transmission, the cost is high, and noise immunity is poor. Early legacy printer interfaces typically used parallel communication.
This relates to the timing control method of data transmission.
Asynchronous Communication: Also known as start-stop mode. When sending character data, a start bit must be sent first, followed by the character data itself, and finally a stop bit. The sender and receiver do not need a unified clock signal; they rely on the start and stop bits to synchronize data frames.
Synchronous Communication: A clock synchronization signal is transmitted simultaneously with the data transmission. The receiver collects data according to a unified clock beat, resulting in higher transmission efficiency suitable for large data volume transfers.
These three modes define the directional capability of data transmission.
a. Simplex
Definition: Data can only be transmitted in one direction and cannot be reversed.
Application: Usually used for simple data output or input scenarios where interactive data exchange is impossible.
b. Full Duplex
Definition: Powerful functionality allowing data to be transmitted in both directions simultaneously. At the same moment, a device can both send and receive data. It’s like a telephone conversation where both parties can speak at the same time.
c. Half Duplex
Definition: Strikes a balance between simplex and full duplex. It allows bidirectional data transmission, but at any given moment, it can only perform either a sending OR a receiving operation, not both simultaneously. It’s like a walkie-talkie: one party must finish speaking (transmitting) before the other can reply (receiving).
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