In order to apply the Ethernet communication technology to the power automation system, this paper analyzes the selection principle of the embedded TCP / IP protocol in detail in conjunction with the power automation system, uses the DSP chip and the network interface control chip to design the Ethernet interface, and introduces the DSP chip Completed the development of embedded TCP / IP system for the control process of the network interface card control chip and the flow of the TCP / IP protocol stack to process data packets. The system can process the data according to the network protocol and realize the Ethernet transmission of the data.
introduction
In recent years, great progress has been made in China's power automation work, and the communication methods of power automation systems have been continuously improved. The earliest is to use RS485 bus to connect various protection and remote equipment together, and use master-slave mode to communicate with the host, the method is simple, and there are many technical defects. The fieldbus technology that appeared later was widely used because of its convenient networking and strong anti-interference capabilities.
There are a variety of fieldbus standards, the most commonly used in power automation systems are LONWORKS bus and CAN bus. The communication rate of LONWORKS bus is 78kbps and 1.25Mbps, and the communication rate of CAN bus is 1Mbps. With the development of the power grid, the limitations of the fieldbus standards are not uniform and the transmission rate is low (the maximum is only about 1Mbps) gradually exposed. Ethernet has the characteristics of strong versatility, mature technology, and rapid increase in bandwidth. Embedded technology appears in the field of industrial control, especially the development of DSP technology and
The wide application of DSP in the field of industrial control makes it possible to use embedded technology to implement Ethernet communication in the design of power automation systems, providing an open, fully distributed, and interoperable communication platform for power automation. The real-time nature of network transmission is a key indicator of the internal communication network of power automation systems. The Ethernet bandwidth reaches 10Mb / s, which can bear a large network load. In addition to the use of switched intelligent hubs, 10Mb / s Ethernet is selected as the internal communication network of the substation automation system, and its real-time network transmission is guaranteed. This article will focus on the selection of embedded TCP / IP protocol and DSP implementation.
The choice of embedded protocol
The TCP / IP protocol is a combination of multiple protocols at different levels, and is generally considered to be a four-layer protocol system that includes a link layer, a network layer, a transport layer, and an application layer [2]. The embedded system is a special system designed to complete a specific function. Embedded systems do not require (or are impossible) to implement all TCP / IP protocols, so embedded TCP / IP is a set of protocols formed by selecting TCP / IP protocol families.
Link layer protocol selection
Selection of link layer protocol The most basic function of the link layer is to transform the physical connection of the original transmission bit stream provided by the physical layer into a logically error-free data connection, and send and receive data packets for its upper layer protocol. Ethernet follows the CSMA / CD (Carrier Sense Multiple Access and Collision Monitoring) protocol stipulated by IEEE802.3, and a universal NIC (network interface controller network interface control) chip can be used to implement the protocol. In addition, the transmission of data on Ethernet is identified by the MAC address of the network, which requires the system to implement the conversion function of the IP address to the MAC address, that is, the ARP (Address Resolution) protocol. The ARP protocol includes two parts: ARP request and ARP response. To communicate with other computers, the system must support ARP response. The ARP request establishes a mapping from an IP address to a MAC address locally to ensure that the external communication is targeted. RARP (Reverse Address Resolution) protocol is mainly used to solve how to get IP address from MAC address, mainly used in diskless workstations. In this embedded system, the IP address can be stored in the local memory without having to obtain the IP address from other servers, so that there is no need to implement the RARP protocol.
Network layer protocol selection
The network layer is mainly responsible for processing data packets in the network. In the TCP / IP protocol family, network layer protocols include IP protocol (Internet Protocol), ICMP protocol (Internet Control Message Protocol), and IGMP protocol (Internet Group Management Protocol). The IP protocol is the core protocol of the TCP / IP family, which makes communication between heterogeneous networks possible. Therefore, the transmission of system data such as RTU across different networks must implement the IP protocol. ICMP stipulates a variety of protocol types and codes. If the full implementation also consumes a lot of system resources, in this embedded system, the network connectivity can be tested in the ICMP protocol, so only the ICMP class Ping response protocol with model 0 and code 0. The IGMP protocol is mainly used to support hosts and routers for multicast. When sending messages to multiple recipients, embedded TCP / IP is used as a special system access network technology. In order to reduce the complexity of processing protocols, you can Directly use the broadcast method, it is unnecessary to use multicast to communicate. Therefore, IGMP is not considered in the design.
Choice of transport layer protocol
The transport layer mainly provides end-to-end communication between the two hosts. The transport layer has two different transmission protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is connection-oriented and provides an end-to-end reliable byte stream on unreliable network services. The TCP protocol has designed a strict 3 times connection establishment handshake process, 4 times connection closure handshake process, and a data transmission process with piggybacking confirmation information and flow control through a sliding window. The UDP protocol is used to provide connectionlessness. It simply sends datagrams from one host to another, but it does not guarantee that the datagram can reach the other end. Reliability must be provided by the application layer. The TCP protocol is used in devices with low real-time requirements such as microcomputer remote control and fault recording to ensure the quality of transmission. The UDP protocol is used in devices with high real-time requirements such as microcomputer protection and automatic control. The inherent shortcomings of low transmission reliability can be compensated by the application layer protocol.
Application layer protocol selection
Application layer protocols mainly refer to user processes. In the power automation system, the international standard telecontrol communication protocol IEC60870-5-103 or IEC60870-5-104 protocol can be used. The draft IEC61850 protocol is an object-oriented and seamless connection communication protocol developed by the IEC (International Electrotechnical Commission) for power system automation. After the agreement is officially announced, it can also be adopted.
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