Siemens S7-1200PLC Network Communication Foundation

Oct 07, 2023

Fundamentals of Network Communication

The communication of PLC includes communication between PLC and PLC, communication between PLC and upper computer, and communication with other intelligent devices. The communication between PLC and PLC is essentially still computer communication, connecting multiple control tasks to form a modular control system.

In the system where PLC and computer are connected through communication, PLC realizes on-site control of work and industry, while computer realizes tasks such as programming, monitoring, and process management, forming a "centralized management, decentralized control" distributed control system (DCS).

The S7-1200 PLC integrates a PROFINET communication interface on its main body, supporting Ethernet and TCP/IP based communication standards. Using this communication interface, communication between S7-1200 PLC and programming devices, communication with HMI touch screens, and communication with other CPUs can be achieved.

This PROFINET physical interface supports 10 (Mbit' s) and 100 (Mbit/s) RJ-45 ports, supporting cable crossover adaptation. Therefore, a standard or crossed Ethernet cable is suitable for this interface.

Serial communication and parallel communication

The data transmission forms of serial communication and parallel communication are completely different. Serial communication is the process of connecting the sender and receiver of data through a pair of wires. During data transmission, one binary bit is sent and received at a time, sequentially on the same wire in a specified order.

Generally speaking, a USB interface is a typical serial communication interface. The characteristics of serial communication are: low efficiency, complex control, fewer cables, and low cost.

Parallel communication refers to the communication of each binary bit of an 8-bit (or 16 bit, 32 bit) data using a separate wire, sending and receiving 8 (or 16 bit, 32 bit) bits at a time, and connecting the sender and receiver in a parallel manner.

Generally speaking, old-fashioned printer interfaces are parallel interfaces. The characteristics of parallel interfaces are: transmitting multiple binary bit data at once per cycle, high efficiency, multiple cables, and high cost.

Asynchronous communication and synchronous communication

Asynchronous communication (asynchronous transmission) and synchronous communication (synchronous transmission) are two basic information transmission methods of serial communication. From an application perspective, the main difference between the two lies in the difference in "frames" in communication methods.

Asynchronous communication is a start stop method. When sending a certain data (8 bits), the first step is to send the start bit, then the data itself, and finally the stop bit (that is, the data is included between the start bit and stop bit). A parity check bit can also be added between the data and the stop bit (optional). The asynchronous communication method has simple hardware and low cost, and is mainly used for data communication with a communication rate below 19.2 kbt/s.

Synchronous communication is the transmission of clock synchronization signals while transmitting data, always collecting data according to a predetermined clock. Synchronous communication has high efficiency, complex hardware, and high cost, and is generally used for data communication with communication rates higher than 20 kbit/s.

Simplex, full duplex, and half duplex

Simplex, full duplex, and half duplex are used to describe the direction of data transmission in communication.

1. Simplex: Data can only be transmitted in one direction. It is generally used for data output and cannot be used for data interaction.

2. Full duplex: Referred to as duplex, data can be transmitted in both directions and can be sent and received at the same time. Usually connected by two pairs of twisted pairs, the communication cost is high, and RS-422 is "full duplex" communication.

3. Half simplex: Similar to full duplex, data can be transmitted in both directions, but at the same time, only data can be sent or received, and cannot be transmitted simultaneously. Usually connected by a pair of twisted pairs, the communication cost is lower compared to full duplex. When RS-485 only uses a pair of twisted pairs, it is called "half duplex" communication.

Common Terminology for PLC Communication Networks

There are many terms and phrases in PLC communication networks, and common terms are as follows:

1. Station. A hardware device that can communicate data and connect external inputs/outputs is called a "station". For example, each PLC is a "station".

2. Master station. A system control station in a PLC communication network that connects data. The master station needs to set the parameters of the entire network. Generally speaking, there is only one master station in each communication network, and the station number is the address (number) of the PLC in the network.

3. Slave station. In the PLC network, all stations except for the "master station" are referred to as "slave stations".

4. Remote device station. A slave station that can handle both bits and words simultaneously.

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