Discussion on communication mode in DCS system
With the improvement of the overall level of automation in China’s industry, the types of control systems and control equipment in enterprises are more and more. At the same time, with the development of the market economy, each enterprise also has higher and higher requirements for the DCS system, in addition to meeting the conventional control, but also requires the DCS system to control the various operating parameters of the system real-time transmission to the upper MIS system or SIS system to carry out data post-processing, sharing, performance optimization. All of these have raised communication problems for the DCS system, including communication with other DCS or PLCS, and communication with the host computer.
In the DCS transformation of four 300MW units (7#, 8#, 9#, 10#) in Jianbi Power Plant, all adopted Foxboro company’s I/A’s system. The devices communicating with I/A’s include 893 types of intelligent data acquisition front end, boiler high wall temperature analysis system, real-time report database system based on PC, PI massive database (SIS system), which covers almost all ways of communication between DCS system and other systems: Use professional communication hardware to achieve communication, use professional communication software to achieve communication, self-developed communication programs (including: serial communication, TCP/IP communication, FTP communication, TCP/ IP-based MODBUS protocol communication).
Below, will make a brief introduction and comparison of the implementation of the above communication methods, and give some suggestions when choosing communication methods.
Two. The use of hardware to achieve communication
For A-B PLC products widely used in the market, Foxboro company specializes in the development of special communication hardware – integrated in I/A’s AB-STATION, through AB-STATION, in I/A’s can directly manage A-B PLC, configuration, communication and data acquisition monitoring. The whole system not only has the advantages of PLC fast, but also has the advantages of DCS friendly man-machine interface and powerful database management.
For the widely used MODBUS protocol (including ASCII protocol and RTU protocol) in the industry, Foxboro company has developed a variety of hardware that conforms to the MODBUS protocol, mainly: MODBUSGATEWAY MODBUSGATEWAYPLUS, applicable to the RS – 232 communication and RS – 485 communication, and can choose as a servant or master. With special communication hardware, it can communicate with all equipment that provides MODBUS protocol communication interface, including DCS systems and PLC products of other companies. This approach is used in most of the communication projects implemented by Foxboro Shanghai.
In addition to the above two kinds of special communications hardware, Foxboro company has a custom protocol communications hardware – FOREIGNDEVICEINTIGRATOR30 equipment integrator (30). Using this communication hardware, the two parties need to communicate as long as they define the communication packet format (including packet header information, data format, packet tail information) and communication parameters (including start bit, stop bit, data bit, baud rate) to achieve communication. In the Zhenjiang Power Plant project, Shanghai Foxboro Company adopted this method to achieve communication with GEPLC and Wuxi Yangshan intelligent data acquisition front-end, greatly reducing the user’s investment.
In addition, for the most commonly used PLC and DCS products on the market, Foxboro provides specialized communication software for communication.
The hardware used in all the above communication methods is the same: communication processor 30 (Communication Process 30), the difference is that the communication software selected is not the same.
For MODBUS, the baud rate used is usually 9600bps, so the number of packets that can be communicated per second is limited.
For MODBUS protocol, if the MODBUS device is different, the amount of communication packet can communicate is not the same, usually used 548 devices can communicate to 256 bytes, that is, 128 analog quantities or 2048 digital quantities.
The standard MODBUS communication frame format is: < server address > < function code > < start address > < data transfer volume > < Check code >. Have different MODBUS addresses for different devices; The same device, different data types (external register, internal register, external hold coil, internal hold coil, etc.) have different function codes; The same data type has different function codes when the read/write operations are different.
Therefore, when considering MODBUS protocol communication, we can not only look at how many analog/digital quantities, but must also consider: how many devices, how many analog/digital quantities of communication data for each device, whether the data is bidirectional transmission, and how much communication rate. After considering the above factors, calculate how many communication processors are needed.
It should be emphasized that: The hardware communication product COMM30 of Foxboro Company is different from the hardware communication mode of many companies by using the serial communication card inserted on the main board of the engineer station/operator station. The COMM30 of Foxboro company has a dedicated CPU chip, which runs independently from the engineer station and operator station, and is an independent processing station in the control network. The normal operation of communication can be ensured even when the engineer station/operator station is down, which is particularly important in the communication involved in the control (such as with schedule control, soot blowing control, etc.).
3. Use professional communication software to achieve communication
Professional communication software is mainly divided into two categories: one realizes real-time data acquisition and transmission, and the other realizes real-time transmission of process images.
3.1 Real-time data collection and transmission
Using the communication software developed by Foxboro or using the communication software developed by A third party for I/A’s is a very convenient way to achieve communication, which can greatly shorten the project cycle.
Foxboro company’s professional communication software from the early DataforWindow, PIMS DataLink to the current use of AIM*AT software adhering to the consistent idea, using AISAPI (FOXAPI early version), FOXAPI internal function call to achieve real-time data reading and writing; Using TCP/IP to realize data bidirectional transmission between I/A’s and PC; Use DDE technology to display I/A’s data in programs that support DDE technology (such as Excel, Lotus1-2-3, Delphi, etc.), and provide VB call functions to facilitate users’ secondary development. The maximum transfer speed of the software is 0.1 seconds. This approach has been adopted more frequently in current projects.
For the current more popular OPC communication protocol, Foxboro company provides specialized OPC communication software, but also can choose a third party, such as: MATRICON company OPC communication software.
The third party communication software is mainly PI real-time database of American OSI company. OSI’s PI system is based on the C/S structure, which can transfer real-time data from more than 100 DCS systems or PLCS to a WindowNT server, using its patented “spiral door compression” technology to compress the data, so that its storage cycle can be up to several years (depending on the hard disk capacity). At the same time, it uses its powerful system tools to realize secondary development, including process screen display, trend display, optimization processing, performance calculation and so on. The PI real-time database application in Foxboro’s I/A’s has had hundreds of successful uses abroad. Jianbi Power plant uses PI system to realize real-time transmission of up to 20,000 points of data of its four 300MW units #7, #8, #9 and #10, up to 2 years of data storage, and uses its powerful secondary development tools to provide application development services for the whole plant MIS (or SIS) system.
Professional communication software, such as: PI, in addition to high rate, high speed, long data storage time, safety and reliability, powerful, friendly interface, easy to use and other advantages, usually in the data transmission also has the following characteristics:
Use “exception report” mode: set the change range of data to be transmitted, and do not transmit the data that does not exceed the change range. This mechanism greatly reduces the communication load.
l Has a “data cache” mechanism: when the network communication is interrupted, the data that needs to be transmitted is cached on the hard disk of the workstation, and when the network communication is restored, the data is transmitted to the database when the network communication is idle. This mechanism ensures that data is not lost.