Industrial control applications rely heavily on classical control theory. With the widespread use of computer technology in industrial control, the requirements of feedback control are reflected in how control commands reach the actuators and how the control effects are transmitted back to the control system via sensors. In complex industrial environments, fieldbus enables accurate, fast, and stable communication, ensuring that the industrial control system receives timely and accurate field data, processes it efficiently, and issues execution commands quickly and precisely.
1. Basic Model of Communication
Communication is the transmission of information from one point to another, such as telephone, radio, and television. Industrial communication systems are similar, requiring transmitters, receivers, and communication links. Link types include twisted-pair, coaxial cable, fiber optic, wireless, and microwave. At the transmitting end, a demodulator modulates the data, and at the receiving end, a modem demodulates the signal to reproduce the original data; the rules governing this communication are called protocols.
Basic model of communication system
2. Special requirements for industrial communication
Industrial communication has some special requirements compared to general communication; for example, the industrial environment is different from the office environment.
Industrial communication mainly involves process data, state variables, and maintenance data. The data classification is shown in the table below:
Time-related characteristics in communication include real-time performance, frequency, jitter, lifespan, response time, synchronization, and temporal and spatial correlation or consistency. Industrial communication often requires ensuring real-time performance, that is, whether the response time meets time constraints.
3. How can fieldbus meet the requirements of industrial communication?
The OSI model defines a framework, the physical layer, which is often confused with physical media such as cables, connectors, network interface cards, and wireless transmission hardware. The physical layer not only defines the physical media and the interface requirements for proper connection, but also defines encoding methods, verification methods, bandwidth, spectrum, carrier wave, baud rate, electrical, optical, and radio signals, control of incoming asynchronous serial communication, cable types, and connector mechanical design. Signals travel from the transmitter to the receiver through the transmission medium. The mechanical and electrical characteristics of the communication medium are also considered.
The data link layer is responsible for creating, transmitting, and receiving data packets. Fieldbus further subdivides the data link layer into two layers: LLC and MAC. The former provides the interface network layer protocol and controls the logical communication with its peers, while the latter provides access to and transmission of specific physical codes.
The application layer specifies the way data is exchanged and defines various information, states, and parameters of the device.
Fieldbus communication data is generally divided into identification data and general messages. Identification data refers to the transmission of process variables between the controller and sensors, between the controller and actuators, and between controllers. General messages are used for downloading and uploading files during system configuration and maintenance phases.
3.1. Alternative wiring scheme for 4-20mA
In the past, industrial communication used analog signals, such as the well-known 4-20mA technology. Later, fieldbus, using digital communication, demonstrated many advantages, including noise immunity, reduced cabling, and diagnostic functions. Therefore, fieldbus will replace 4-20mA.
One of the key requirements for adopting fieldbus is distributed intelligence. In order to access the fieldbus, intelligent sensors and actuators need to have computing power, digital communication, and protocol standards, which results in relatively high equipment costs.
3.2 RS-232 and RS-485 Electrical Standards
Some people may consider RS-232 and RS-485 as buses, but they are actually the same interface standard. Interface standards are mainly described in terms of electrical characteristics (signals), mechanical characteristics (DB9), and pin functions. The main difference lies in how the logic is represented: RS-232 uses voltage to represent 0 and 1, while RS-485 uses voltage difference to represent 0 and 1. A detailed comparison is shown in the table.
Because RS-232 can only achieve point-to-point communication (two-machine interconnection) and cannot network (multi-machine), while RS-485 can be connected into a bus network, and many fieldbuses are based on the RS-485 standard, some people may understand it as a 485 bus. However, strictly speaking, the 485 bus is not a fieldbus. It is just that some fieldbuses use RS-485 as their physical layer definition. Therefore, fieldbus and 485 are not concepts at the same level.
3.3 Manchester encoding, encoding, and code elements
Pin assignments for cables and connectors. Pin assignments depend on the cable type and the network architecture used. Signal format. The encoding used for signals 0 and 1, and the transmission of specific values in the form of values or analog signals, depends on the network architecture used. Most fieldbuses use Manchester encoding for communication.
The clock synchronization signal is hidden within the data waveform. Each symbol contains a transition; low to high or high to low represents a symbol. First, understand the concepts of symbol, bit, baud rate, and bit rate. A symbol is like a word, and a bit is like a letter. The number of symbols transmitted per unit time is the baud rate, and the number of bits transmitted per unit time is the bit rate. A word can consist of one letter or several letters, meaning the number of symbols is a multiple of the number of bits. In this case, the baud rate is a multiple of the bit rate. We all have experience memorizing words. For example, if we memorize 10 words a day, assuming each word consists of 4 letters, then the baud rate is the number of words memorized each day, and the bit rate is the total number of letters in all the words memorized each day.
3.4 Communication Medium
Fieldbus typically uses wired connections, operating on the principle of electromagnetic waves propagating through a communication medium. These media include twisted-pair cables, coaxial cables, and optical fibers, as shown in the figure. However, with the evolving wiring requirements in industrial settings and the rapid development of wireless technology, fieldbus is increasingly adopting wireless connections.
3.5. Communication model
Communication patterns refer to the ways in which two or more applications interact with each other, and are divided into two categories: client/server and producer/consumer (publisher/subscriber) patterns.
3.5.1 Client/Server Mode
The diagram illustrates a client/server model, which is well-suited for transmitting state data. When a client sends a read request, the server responds. The communication process in a client/server model generally involves four steps: request, indication, response, and acknowledgment. An indication means the server has received a request event, and an acknowledgment indicates the client has received a response. Responses can have different meanings; some indicate message receipt, while others indicate service initiation or result return. For a read service, the value of the read object is included in the response. The read request includes the object name and accesses the local object using an addressing mechanism. The response either returns the result or the reason for failure, along with the response time. The object can be a priori, a simple variable, or a complex structure.
Protocols conforming to this model are mostly derived from the Manufacturing Message Specification (MMS). The application layer manages various objects based on the MMS model, such as tasks (creation, cancellation, start, resumption, and stop), variables (read and write), and domains (download and upload). MMS typically only has a subset of services. The entire operation is divided into three parts: request, execution, and response, which are related to the MAC layer processing time and service execution time.
Fieldbuses such as WorldFIP, ROBIBUS-FMS, PROFIUBUS-DP, INTERBUS, AS-I, and P-NET all use a client/server communication model. BatiBus, however, is a special client/server model because it lacks response and acknowledgment functionality.
3.5.2 Publisher/Subscriber Pattern
It includes a publisher application and several user applications, and operates in two modes: "push" and "pull". In pull mode, the publisher manager sends a request, and the publisher responds via broadcast (or multicast); while in push mode, a subscriber sends a request, the publisher responds to the request first, and then sends broadcast (or multicast) data. As shown in the diagram, push mode involves an additional step.
The publisher/subscriber model is well-suited for transmitting event data and can be used for "event notification" requests and prompts, as well as prompts defined in MMS messages. The publisher/subscriber model is used for communication between buffers (read-write services). WorldFIP, CAN, LonWorks, EIBus, ControlNet, SWIFTNET, and FF all utilize the publisher/subscriber model.
3.6 Communication Mechanism
Communication mechanisms include periodic communication, response mechanisms, and triggering mechanisms. Periodic communication originates from sampling theory, the foundation of automatic control and event detection. Most tagged data serves as input or output to control algorithms, requiring periodic transmission, primarily used in centralized control systems for periodic polling operations. The communication periods for different data may differ, causing jitter; therefore, communication protocols aim to minimize jitter. These systems are based on state communication and are sometimes called "time-triggered systems."
The diagram illustrates an example of periodic communication. Each basic cycle has C and D, every two cycles have B and E, and every three cycles have F. Therefore, the bus cycle (large cycle) is equal to the least common multiple (LCM) of all cycles, while the tick (small cycle) is a time interval equal to the greatest common divisor (HCD). Some fieldbuses transmit all data periodically, but the network load becomes too high. Therefore, some data is transmitted aperiodically; for example, some state variables can be transmitted only after they change.
The protocol introduces an acknowledgment mechanism, whereby the receiver notifies the transmitter whether a message has been received correctly. In fieldbus applications, non-periodic communication requires an acknowledgment mechanism, while periodic communication does not, because in periodic communication, if an error occurs, the receiver will ignore the error and wait for subsequent correct data.
In addition to periodic and non-periodic communication, there is also time- and event-triggered communication. Most fieldbuses tend to be time-triggered systems, and some also combine event triggering, with events managed by a periodic server.
3.7 Error Control and Flow Control
Error management and recovery mechanisms must be within user control, i.e., within the application process. Error detection or communication control is implemented either by the sender or the receiver. In fieldbuses, both scenarios exist, and fieldbuses typically provide both communication mechanisms.
