Abstract: This article focuses on the Industrial Ethernet Modbus protocol for realizing control and data communication between DCS and PLC. Keywords: DCS system; PLC; Industrial Ethernet; Modbus protocol 1. Overview of DCS System Distributed Control System (DCS) refers to a system where control and field operations are distributed, installations are geographically dispersed, and management and display are centralized. In the field of automation, more and more enterprises have established comprehensive automation management platforms encompassing various levels, from factory field equipment to control and management layers, and have established enterprise information platforms based on industrial control network technology. 2. PLC System PLC evolved from early relay logic control systems combined with microcomputer technology. It is an industrial control instrument based on a microprocessor, integrating computer technology, control technology, and communication technology. It combines sequential control, process control, and data processing, offering high reliability, powerful functions, flexible control, and simple operation and maintenance. 3. Network Communication Technology Industrial Ethernet is technically compatible with commercial Ethernet, but it meets the needs of industrial sites in terms of environmental adaptability, reliability, security, and installation and use. It has a series of advantages, specifically: Ethernet technology is widely used and supported by all programming languages; it has abundant hardware and software resources; it is easy to connect to the Internet and realize seamless connection between office automation networks and industrial control networks; it has a large space for sustainable development, etc. It provides Ethernet standards for data transmission of control networks. The main ones are: (1) 10Base-T Ethernet coaxial cable, 10Mbit/s; (2) Fast Ethernet-optical cable or twisted pair, 100Mbit/s; (3) Gigabit Ethernet-optical cable or twisted pair, 1Gbit/s; (4) 10Gigabit/kGigabit Ethernet-supports WAN and MAN. It uses communication networks as a link, and the network structure can be divided into bus type, ring type and star type. DCS adopts bus type network such as I/AS. It adopts network communication technology. This is the key technology of DCS. It makes control and management real-time and solves the problem of system expansion and upgrade. The data communication and control between DCS and PLC is accomplished through network communication technology. 4 Modbus Protocol The Modbus protocol is a typical industrial Ethernet. It was originally the communication protocol for PLC products of Modbus Company in the United States. Due to its wide use, it was later developed into the Modbus TCP/IP specification by the Modbus organization in 2002. The Modbus protocol has extremely wide applications in industrial control. The communication technology between DCS and PLC is based on this protocol. The main contents of the Modbus protocol are as follows: (1) Interface Standard RS485 is an interface standard between a computer or terminal and a modem. It is developed based on the RS232C standard. In many industrial environments, it is required to complete the communication task with the fewest signal lines. The widely used RS485 serial interface bus is precisely to meet this need. The network signal transmission line of RS485 interconnection is a twisted pair. At any given time, only one station can send data, while the other station can only receive data. It can only support half-duplex communication. RS485 is very convenient for multi-station interconnection. In addition to saving expensive signal lines, it can also transmit data at high speed over long distances. (2) Function Code The type of data exchanged between systems that support Modbus is controlled by the function code FC. The function codes that can be used to implement bit-by-bit data exchange are: FCO1 read coil (output) status, FC02 read input status, FC05 force single coil, and FC15 force multiple coil. The function codes that can be used for data exchange between registers are: FC03 read holding register, FC04 read input register, FC06 preset single register, and FC16 preset multiple register. Each function code corresponds to a specific starting address range, and the amount of data transmitted varies depending on the category. (3) Transmission parameters Transmission parameters include transmission rate, data bits, stop bits, and parity bits. In serial communication, each byte is sent bit by bit in a certain order. For asynchronous serial communication, in order to ensure synchronization between the sender and receiver, not only should the transmission rates of the sender and receiver be the same, but auxiliary bits should also be added on top of each data bit. a) Transmission rate (baud rate): refers to the speed of data transmission, and the unit is bits/second (bps). b) Data bits: describes how many bits represent a transmitted character. c) Stop bits: define the minimum possible time interval between transmitting two characters. d) Parity bits: used to judge character transmission errors and protect data. It defines whether the total number of data bits to be transmitted is odd or even. When “NO check” is selected, the check bit is not transmitted, which reduces the security of data transmission. (4) Protocol parameters The main protocol parameters are: slave address, operation mode, and multi-character delay time. a) Slave address: Specifies the Modbus slave address and which CP will respond. One master station can send communication requests to up to 247 slave stations. The Modbus message format is request/response frame mode. Only when the master station’s request instruction string starts with “00” will it be received by all slave stations. b) Operation mode: There are two modes: normal mode and conflict prohibition mode. In the conflict prohibition mode, the received error information is ignored and transmission continues. c) Multi-character delay time: Under normal circumstances, the time interval between characters is very small. For the receiver, if the character interval exceeds the allowable value, it can be considered that the information frame has ended or the transmission is abnormal. 5 Conclusion In practical applications, our plant’s DCS network, 1.2 million t integrated automation dispatch network, newly built 800,000 t dispatch network and various DCS and PLC systems have realized the plant’s full-plant dispatch automation through network communication technology, laying a solid foundation for further improving the company’s production and management level. Click to download: DCS and PLC Communication Technology Analysis