Design and implementation of universal RTU based on embedded Internet technology

Abstract: Aiming at the low communication rate, non-uniform communication protocol and closed system of traditional RTU, this paper analyzes the design of an open, flexible and reliable Webit-GRTU architecture based on the IEC 60870-5-104 communication protocol and combined with embedded Internet technology. The design goal is verified through examples, which opens up a way for low-cost replacement of traditional RTU.

Keywords: embedded Internet; remote terminal equipment; IEC 60870-5-104 communication protocol; TCP/IP; Webit-GRTU

Remote Terminal Unit (RTU) is a remote measurement and control intelligent device. It can not only collect, monitor, calculate and store on-site equipment signals, but also independently complete the predetermined control program, execute the instructions of the host computer, and transmit the execution results and on-site data back to the host computer. RTU has better communication and control capabilities than PLC, and is suitable for harsh working environments and the implementation of complex special algorithms.

However, with the continuous development of automation, computer, network and other technologies and the widespread implementation of ERP strategies, various applications and services have put forward higher and higher requirements for the real-time, reliability and scalability of information. At present, traditional RTUs are difficult to meet the requirements of the actual site in terms of structure and implementation methods. The main manifestations are: (1) The communication protocol is not unified. There are many communication protocols, such as DNP 3.0, Modbus, SCI, CDC, CDT, etc. Major manufacturers follow their own set of communication protocols, and the interconnection and upgrade between different devices are difficult; (2) The communication rate is low. Traditional RTU communication methods are mostly RS-232/422/485 bus methods. The most basic "four remote" quantities can only be transmitted in seconds. The communication rate has become a bottleneck for system performance and is far from meeting the real-time monitoring requirements in the production process.

In the late 1990s, Internet/Intranet technology became increasingly mature, the concepts of equipment networking and ubiquitous computing models continued to take shape, new automation systems and management networks began to widely adopt TCP/IP network technology, and new international standards for automation control and communication were constantly introduced. This requires new RTUs to make full use of existing communication resources, improve data throughput, ensure the real-time performance of the system, and at the same time have better openness and compatibility. In order to adapt to this development trend, this paper follows the IEC 60870-5-104 communication standard of the International Electrotechnical Commission and designs a universal RTU-Webit-GRTU based on embedded Internet technology. It can provide comprehensive remote access to the Webit-GRTU system on any Internet/Intranet node, opening up a way for low-cost replacement of traditional RTUs.

1 Embedded Internet Technology and Webit

Embedded Internet technology is a device access technology or a heterogeneous network interconnection technology. The main problem it solves is to monitor, diagnose, manage and maintain devices and heterogeneous subnets connected to the Internet from different subnets and different physical areas through Web and embedded technology, so that users have the ability to remotely monitor, diagnose and manage various devices or other types of subnets connected to the Internet. Figure 1 is a schematic diagram of Webit using embedded Internet technology and then connected to the Internet after being bound to TVs, cameras, UPS, RTU and other devices. With the help of the Internet, geographical barriers can be broken to achieve global communication and control.

Internet communication protocols have high requirements on the CPU speed and memory capacity of computer systems. There is no difficulty in using them in PC systems, but they must be used in embedded systems with limited resources and must make trade-offs according to needs and reasonably select the implementation and processing solutions of communication protocols. Based on the real-time kernel WebitX 2.0, Webit has made different degrees of reductions to the various layers of the TCP/IP protocol cluster according to the characteristics of embedded systems. It has a compact structure, low cost, reliable performance, and powerful high-speed, open, and real-time network communication capabilities.

2 IEC 60870-5-104 Communication Protocol and TCP/IP Protocol Stack

The IEC 60870-5-104 communication standard is a telecontrol transmission protocol standard for "IEC 60870-5-101 network access using a standard transmission file set".

It is a communication standard formed based on the IEC 60870-5 series of documents, combining the application layer of IEC 60870-5-101 with the transmission function provided by TCP/IP (Transmission Control Protocol/Internet Protocol).

Through this standard, within the framework of TCP/IP, it can be applied to different network types according to actual application needs, including X.25, FR (Frame Relay), ATM (Asynchronous Transfer Mode) and ISDN (Integrated Service Data Network). Figure 2 is a schematic diagram of the standard structure formed by IEC 60870-5-104 combining IEC 60870-5-101 with the TCP/IP protocol cluster: Since IEC60870-5-101 does not use application protocol control information (APCI), the application protocol data unit (APDU) in IEC60870-5-104 is equivalent to the application service data unit (ASDU) in IEC60870-5-101, and IEC60870-5-104 also defines application protocol control information (APCI), so APDU is a combination of ASDU and APCI. The control field of APCI defines control information to prevent message loss and duplication, as well as monitoring of message start, stop and transmission connection.

3 Design and Implementation

3.1 Design Concept

As a remote measurement and control intelligent device, RTU should give full play to the functions of RTU and Webit after binding with Webit, realize transparent interconnection between field equipment and the Internet, and truly integrate equipment and network. Intelligence, simplicity, reliability, universality, and openness are the basic design ideas of this RTU system: (1) Webit-GRTU strictly follows the IEC 60870-5-101/104 communication protocol in communication implementation and is compatible with all standard industrial automation workstation software; it provides technical support such as API, ActiveX, OLE, DDE, OPC, COM, DCOM, ODBC, etc.; it can be easily connected with various automation control systems to form a powerful SCADA system. (2) It adopts a separate architecture for communication and control. Data acquisition and control use an internal expansion bus, which can be flexibly combined according to the complexity of the system to independently complete logical control. The RTU communication part uses a separate microcontroller and downloads different software according to different communication protocols, which is convenient for software upgrades and expansions. (3) It is easy to use.

Through the embedded WEB server, technicians can set all parameters through IE or Netscape browser with the help of prompts; and can simulate on-site conditions through simulation software to facilitate debugging and training. (4) It has strong versatility and can be widely used to control relatively simple ordinary civil facilities as well as complex controls in harsh environments.

3.2 Webit-GRTU Architecture

Webit-GRTU adopts a communication and control separation system structure. The data acquisition and control output part of Webit-GRTU adopts the INTELMCS80C196 chip to complete the functions of remote signaling, remote measurement, electric energy pulse counting and remote control output, and adopts a 16-bit data bus.

The Webit-GRTU communication module is based on the Atmel AT90S8515 microcontroller, equipped with the Realtek RTL8019AS 10M Ethernet interface chip, with RJ-45 interface, 14-bit TTL level bidirectional I/O, UART port and ECP system programming port, as shown in Figure 3. The software structure in the figure mainly includes: real-time kernel WebitX 2.0, thin TCP/IP protocol stack and RSC (Remote System Control). Among them, RSC is mainly used for system parameter setting, power monitoring, remote reset and startup of the system; thin TCP/IP protocol stack includes ARP, IP, ICMP, UDP, TCP, HTTP, TFTP, SNMP and other protocols. In addition, an external SEEPROM memory is set up to store user data, such as system parameters, HTML web pages, GIF and JPEG images, etc., which are managed by the file system of WebitX 2.0. The system has RS-232 and RS-485 serial communication interfaces, with data transmission rates selectable between 300 and 57,600, and communicates in accordance with the IEC 60870-5-101 communication protocol; the Ethernet communication rate is 10 Mbps, supports standard TCP/IP protocol, and communicates in accordance with the IEC 60870-5-104 communication protocol.

The data exchange between the communication module and the data acquisition control module is carried out through the public memory area opened by IDT7130. This is a high-speed dual-port static RAM with a bus arbitration circuit on the chip and two independent control and address buses, which is suitable for fast bidirectional transmission of large amounts of data between CPUs.

The public memory area is mainly used to store RTU telesignaling, telemetering and other four remote information.

4 Typical Applications

Figure 4 is a network topology diagram of the LF (Laddle Furnace) refining furnace control system in a steel plant, which is a typical application of Webit-GRTU (since the control system is relatively large and complex, this article mainly focuses on the control part of Webit-GRTU).

4.1 Process

Webit-GRTU controls the wire feeder part of the system, which is in the same TCP/IP local area network as the LF furnace body control PLC, feeding control PLC and operator station. During the molten steel refining process, the LF furnace must continuously add metal composite materials through the wire feeder. The amount of metal added varies depending on the type and component content of the molten steel, and requires continuous adjustment as the refining time goes by. The operating site has high temperature, strong arc interference and harsh environment, which is not suitable for manual on-site operation.

On the one hand, Webit-GRTU itself can realize intelligent control and manual control of the wire feeder through built-in software, and adjust the wire feeding speed and time interval of the wire feeder. On the other hand, Webit-GRTU needs to maintain communication with the LF furnace body control system, continuously receive molten steel type, composition, refining time and other related parameters and instructions, and optimize the control process of the wire feeder.

4.2 Performance Analysis

The master station communication in this system all adopts TCP/IP network structure, and the distributed I/O adopts PROFIBUS fieldbus structure. Webit-GRTU, Intouch monitoring system, Siemens PLC S7-400, etc. can work together and exchange data well without intermediate conversion devices. Among them, Webit-GRTU has exerted independent intelligent control and coordination capabilities, and has withstood the test of the harsh environment of the steel plant.

Compared with the original RTU using Modbus communication, each operation station and server receiving RTU data must be configured with a special Modbus communication module and communication software. However, using Webit-GRTU, it is very convenient to integrate RTU control and PLC control system through ActiveX, DDE, and OPC, without additional software and hardware investment. Webit-GRTU uses RTL8019AS 10M Ethernet card. After testing, the single-byte transmission rate is generally around 720μs, which is about 30 to 100 times faster than the traditional RTU serial transmission method. In actual applications, the data scanning cycle of the operation station screen is reduced from the original 5 to 10 s to less than 2 s, which fully meets the process control requirements. At the same time, Webit-GRTU breaks through the limitation of transmission distance. Through gateways and routing, information can be transmitted to any corner of the Internet.

5 Conclusion

Webit-GRTU not only complies with the IEC 60870-5-101/104 communication protocol, which solves the interconnection and upgrade problems between devices, but also increases the communication rate of RTU to microseconds to ensure the real-time performance of the system. Practice has proved that Webit-GRTU has simple network wiring, good scalability, and strong anti-interference ability. It provides a reference for the low-cost replacement of traditional RTUs, covers all intelligent devices through a unified Internet/Intranet network, and opens up an effective way to realize the transparent factory concept of integrated management and control.