Abstract: This paper discusses the basic concepts of connection conversion bridges for industrial control systems and the basic requirements of industrial control systems for bridges. Based on this, the concepts of conversion bridges and network conversion are proposed, and the basic requirements of industrial control systems and fieldbus technology for bridges and network conversion are given. Through discussion, three bus system connection types designed using the MC68HC05C8 are proposed, and the basic structures of bridge modules and network conversions are presented for these three connection types.
Keywords: Fieldbus communication, industrial control bridge. In recent years, with the advancement and development of fieldbus technology, the concept of industrial control system connection bridges and corresponding products have emerged. It should be said that the emergence of the bridge concept and products marks the beginning of solving the problems of "open" systems, and also the beginning of fully realizing information-based fieldbus technology. Currently, the role of bridge devices in industrial control fieldbuses is to realize the connection between devices with different signal transmission modes, especially the physical layer connection; however, a bridge is not merely a physical interface converter, but must also have the function of communication protocol conversion. Based on the content of fieldbus technology and the communication structure of industrial control systems, this paper proposes the basic functions that industrial control system bridges must achieve. These functions are actually the basic requirements of industrial control systems for bus bridges.
Physical interface conversion primarily enables signal mode conversion and is also fundamental to communication protocol conversion. Building upon physical interface conversion, communication protocol conversion provides for the conversion of data connections and communication modes between different buses. Of these three conversion functions, operational information conversion is the most complex and generally requires a dedicated conversion control platform. Based on a discussion of the basic technical requirements of network bridges, this paper designs three network bridge implementation schemes. These three network bridge schemes are, in fact, the basic devices for information network conversion in control systems.
1. Basic Concepts of Switching Bridges
For a control system using different fieldbus technologies, the connections between them can be divided into the following three types: (1) Loose connection. This type of connection refers to the fact that the fieldbus systems to be connected are independent of each other and do not need to strictly process control information in real time, but only need to transmit some system information. This type of connection is relatively easy to implement. Just connect the two systems to the information system respectively and set up the corresponding management platform.
(2) Tight Connection. If devices using two different bus technologies are used in the same control system, this is called a tight connection. The characteristic of a tight connection is that the devices are independent of each other, but the system must achieve a unified bus structure. Undoubtedly, a dedicated bridge must be used in a tight connection. (3) Dependent Connection. Connecting two or more existing independent control systems together to form a new, complete control system is called a dependent connection. The characteristic of a dependent connection is that the device buses form a grouped form, which requires a dedicated bridge. It can be seen that the dependent connection is a connection type very similar to the tight connection. From the above three connection types, it can be seen that, except for the loose connection, all require a dedicated bridge to realize the bus connection. In addition, for devices that do not use fieldbus technology, when connecting to a fieldbus system or connecting themselves to form a bus system, a dedicated bridge device is also required.
Based on the fieldbus technology discussed above and the communication structure of industrial control systems, the basic functions that an industrial control system bridge must implement can be proposed. These functions represent the basic requirements of an industrial control system for a bus bridge: (1) It must have a physical interface conversion function; (2) It must have a communication protocol conversion function; (3) It must have an operation information conversion function. The physical interface conversion mainly realizes the signal mode conversion, and it is also the basis for the communication protocol conversion. Based on the physical interface conversion, the communication protocol conversion provides the conversion of data connection and communication format between different buses. Among these three conversion functions, the operation information conversion function is the most complex to implement and generally requires a dedicated conversion control platform.
2. Basic characteristics and structure of bus bridges
In fact, the basic functional requirements that fieldbus systems place on bridges are also the basic characteristics of bus conversion bridges. Based on the three basic requirements mentioned above, this paper proposes the basic structures of different bus bridges.
(1) Basic Structure of a Compact Bridge. For compact conversion requirements, a bridge is actually a functional module attached to devices in different bus systems. That is to say, this type of bridge is a necessary additional device for bus devices, and its module structure is shown in Figure 1. As can be seen from Figure 1, in order to connect devices on bus A to the bus system B, the bridge module should contain bus modules for both bus A and bus B, and connect devices on bus A and bus B through a protocol conversion layer. For devices on bus A, it is still connected to bus A; and for devices on bus B, it is still connected to devices on bus B.
(2) Basic Structure of Dependent Bridges. In dependent connections, the bridge's role is to realize information connection and management between different bus systems. Compared with tightly coupled bridges, dependent bridges are actually complex conversion systems, but they do not need to be equipped with a bridge module for each device. The conversion system can be treated as an information device for each bus system. The structure of this network conversion is shown in Figure 2. As can be seen from Figure 2, both bus systems treat each other as a device of their own system. Therefore, this network conversion is transparent to both bus systems.
(3) Communication Module Conversion Bridge. In addition to the bridges and network converters between different fieldbus systems mentioned above, industrial systems currently require a bridge for converting between different signal modes and types. Using this type of bridge, devices with different signal modes can be connected to a single system. If combined with a fieldbus device module, the existing equipment and system can be directly upgraded to a fieldbus control system. Compared to the previous two types of bridges and network converters, this type of bridge is simpler, essentially functioning as a special signal exchanger. Most bridge products on the market currently fall into this category. This article mainly discusses the structural design of this type of bridge.
3. Three Design Schemes for Communication Mode Conversion Bridges
To address the aforementioned communication mode conversion bridges, this paper proposes three communication mode conversion bridge structures.
(1) Overall Implementation Scheme. Based on the basic functional requirements of the communication mode conversion bridge, this scheme adopts the method of implementing different communication protocol conversion on a single circuit board, as shown in Figure 3. The control core of the bridge is the MC68HC05C8 microcontroller, which uses its parallel I/O ports A and B to form an IEEE488 interface circuit; the high 4 bits of I/O port C are used as the input of the 3/8 decoder, and the PC3 bit of port C is used to control the reception and search of the HART protocol circuit. The function of I/O port D is to implement SPI (synchronous serial communication interface) and SCI (asynchronous serial communication interface). The SCI port can select two channels through the decoder, one for communication with the host computer and the other for connecting the HART circuit; the SPI port can select two RS-485 circuits, one RS232 circuit and E2PROM through the decoder. The E2PROM is used to save the bridge's setting status and record important interface control parameters. After system maintenance, power failure or other unexpected events, the bridge can continue to work as long as it is powered on again without having to be set and adjusted again. The advantage of this solution is its compact circuit structure, making it suitable for smaller networks. However, its disadvantage is that the bridge is a fixed, integrated structure, which limits its flexibility.
(2) Interface Module Circuit Combination Scheme. The basic structure of the modular bridge is shown in Figure 4. The modular design scheme allows the entire bridge to be composed of different physical interface module circuits according to the actual needs of the project. This structure is more suitable for occasions with many types of communication protocols and has considerable flexibility. It can connect four types of devices: RS-232, RS-485, 4-20mA current loop and HART devices. Each device forms a module, which enables any two non-intelligent field devices with different interface standards to communicate with each other and make them intelligent to a certain extent. In the modular circuit structure, the bridge control circuit remains unchanged. The SCI bus part is only used for RS-232 communication with the host computer, while RS-485 and HART are designed using a local module circuit method. In addition, considering that IEEE-488 is not widely used in the field, while the 4-20mA current loop still has a large application in the field, it was decided to add a 4-20mA current loop module and delete the parallel port. All modules are connected to the bridge control circuit by plug-in connection. Because the parallel interface has been eliminated, the microcontroller offers a wider selection of interface modules. This paper uses I/O ports A and B to directly provide module selection signals, allowing up to 16 modules to be connected. It is worth noting that since the bridge designed in this paper uses a controller switching method, the number of connected modules should be determined by the on-site requirements for data transmission intervals.
(3) Intelligent Module Combination Scheme. The two schemes mentioned above have basically the same suppression technology characteristics. Since using a single microcontroller to control the conversion of multiple interfaces will inevitably impose various technical performance limitations on the system. To improve the intelligence of the conversion bridge, a third design scheme is proposed here: intelligent module circuit combination scheme. This scheme uses intelligent modules to implement the physical interfaces of various types of communication protocols. Each small module uses a separate microcontroller, giving the bridge the characteristics of intelligent communication protocol interfaces. This intelligent module scheme improves the intelligence level of the bridge, thus greatly improving the overall technical performance of the bridge. The system structure is shown in Figure 5.
The third scheme adopts an SPI system bus plug-in card structure. The system consists of a main control board and various intelligent plug-in boards. The main control board has many sockets, allowing for easy plugging and unplugging as needed in the field. The main control board's task is to manage the entire system via the SPI bus, providing the intelligent services (such as parameter connection and conversion) required by each plug-in board. Each plug-in board is an independent microcontroller system, connected to the main control board via the SPI bus. Each plug-in board is equipped with corresponding interface circuits. Based on the main control board, communication interface conversion and protocol conversion can be implemented between plug-in boards, within plug-in boards, and between plug-in boards themselves.
4. Conclusion
Through discussion, this paper proposes that control systems using different fieldbus technologies can be categorized into three types of connections: loose, tight, and dependent. Except for the loose type, all three require a dedicated bridge for bus connection. Furthermore, for devices not using fieldbus technology, a dedicated bridge is also needed when connecting to a fieldbus system or forming a bus system independently. The basic functions that an industrial control system bridge must perform are physical interface conversion, communication protocol conversion, and operational information conversion. Physical interface conversion primarily realizes signal mode conversion and is also the basis for communication protocol conversion. Based on physical interface conversion, communication protocol conversion provides conversion of data connections and communication formats between buses. Among these three conversion functions, operational information conversion is the most complex to implement and generally requires a dedicated conversion control platform.
