Overview:
CANBus bus is a kind of supports distributed real-time control system of serial communication in the local area network. Due to its high performance, high reliability, good real-time performance and its unique design, has been widely used in the detection and control system of data communication between the actuator. With the development of the power grid, the substation integrated automation proportion is more and more big, the communication between monitor and protect the reliability and accuracy of is more and more important. The CANBus bus communication rate can fully meet the requirement of real-time measurement and control system, the error rate is extremely low, running effect is very good. At the same time because of using the bus technology, greatly improve the reliability of the system, real-time, and high cost performance, simple and convenient installation and maintenance, can create the good economic benefits, thus has broad application prospects.
1, The introduction:
CAN bus is English (Controller Area Net), the abbreviation of it with the traditional BITBUS belong to bus communication network, but there are essential differences between both. Previous domestic many users since there is no better choice, all adopt BITBUS as a bridge of communication, its characteristic is: 1) the master-slave structure only one master node on the network, the rest are from the node. As a result of its damage: a BITBUS only one master node on the network, cannot constitute a multi master Yu Jiegou system, thus particularly high reliability requirements for the master node, otherwise once the master node fails, the entire system will be paralyzed; 2) the command response data communications model, a data transmission network administration is made by the master node command, after received orders from node in response to the master node, this characteristic makes the data on the network transmission efficiency is greatly reduced, and the master node controller is very busy; Bottom anomalies at the same time, the data can't upload immediately, must wait for the master node issued an order, poor flexibility, in many real-time demand higher occasion, this is the Achilles' heel, is likely to cause a major accident; 3) BITBUS and there is the bottleneck of data exchange between PC phenomenon. In spite of the high data transfer rate of BITBUS itself up to 2.4 MBPS, but because of the PC interface board PCX344 board with PC bus is the data exchange between a one byte, so as to make the data exchange between PCX344 and PC bus rate is very low, actually result in serious bottlenecks, greatly affect the speed of data exchange; 4) BITBUS physical layer USES is older RS422/485 specification, the link layer as the SDLC protocol, overall low efficiency, poor flexibility, especially its error handling ability is not strong.
Users have adopted CANBUS bus technology, makes got well solve the above problems. CAN any node in the network CAN be as the main node active exchange data with other nodes, thoroughly solved the BITBUS has long been bothering people from node to the problem of data exchange with other node on their own initiative, and therefore to the user's system design offers great flexibility and CAN greatly improve the performance of the system. CAN in the network nodes CAN be divided into priority, for the real-time requirement of the user is a Gospel, which is BITBUS incomparable. CANBUS, moreover, the physical layer and link layer USES the unique practical technology, make its in anti-interference, error detection ability much more than the characters of BITBUS, it's worth noting that we develop CANBUS interface board is in fully learned BITBUS interface board on the basis of the shortage of the elaborate design, with the main engine rate of data exchange, data processing ability, anti-interference performance is far more than BITBUS products, etc.
Two key features:
(1) the ISA bus card, can be arbitrary in XT, ats compatibles, easily constitute a distributed monitoring system;
(2) in accordance with CAN bus specification 2.0 B;
(3) the CANBUS can master more works, any one node on the network can at any time the initiative to send information to the other nodes on the network, and regardless of master-slave, flexible communication way. Using this feature can be easily constitute more machine for subsystem;
(4) the CANBUS nodes on the network can be divided into different priorities and can satisfy the real-time requirement of different;
(5) the CANBUS with non-destructive bus arbitration technique, when two nodes to transmit data information on the network at the same time, take the initiative to stop the low priority node data, and the high priority nodes may continue to transmit data, the affected them greatly save the time of the bus conflict decision;
(6) CANBUS can point to point, point to multi-point (group) and global broadcasts several ways of sending and receiving data;
(7) of the CANBUS communications directly furthest distance up to 10 km / 5 kBPS;
(8) the number of nodes on the CANBUS actual can reach 110, theoretically could reach 2000;
(9) CANBUS adopts short frame structure, the effective number of bytes in each frame eight, such transfer time is short, low interference probability, to send and receive short time;
(10) CANBUS every frame information has error detection CRC check and other measures to ensure the data error rate is extremely low;
(11) communications medium using cheap twisted pair can, no special requirements;
(12) in the CAN node error serious cases, has the function of automatically shut off the bus, cut off its connection with the bus, in order to make the other bus operation is not affected;
(13) NRZ encoding/decoding way, using a filler (insert) technology;
(14) the user interface is simple, easy to programming, it is easy to constitute the user's system;
The transceiver on the (15) CAN bus interface card with the host using photoelectric isolating circuit, in order to improve the anti-jamming ability and reliability of the system.
CAN bus is a kind of supports distributed real-time control system of serial communication in the local area network, due to its high performance, high reliability, good real-time performance and its unique design, has been widely used in the detection and control system of actuators, the data communication between the compared with the traditional DCS, the CAN bus has the following some prominent advantages: (1) the bus type structure. A pair of transmission lines (bus) can be articulated many sets of field devices, two-way transmission multiple digital signal, this structure than one-on-one one-way analog signal wiring is simple structure, low cost of installation, maintenance is simple; (2) open interoperability. Field bus adopts unified agreement standard, is the Internet network open, is transparent to the user. In the traditional communication system, different manufacturers of equipment CAN't access to each other, and CAN use uniform standard, different manufacturers products CAN easily access the same network, integrated in the same control systems to interoperate, thus simplifying the system integration; (3) complete distributed control. Fieldbus control function to field intelligent instrument and equipment as the network node, did it completely decentralized control, improved the system flexibility, autonomy and safety and reliability, reduce the calculation burden of the DCS control station CPU; (4) high reliability. Adopting digital signal transmission data and improve the data accuracy and anti-interference. Will control function in the field devices, scatter risk and increased the reliability of the system; (5) information integrated, flexible configuration. Through the digital transmission field data, the CAN bus CAN get the various states of field instruments, diagnostic information, realize the real-time monitoring and management system.
Three specific application:
With the development of the power grid, the substation integrated automation proportion is more and more big, the monitor, the communication protocol between the protection is more and more important, especially the function of remote control require very high reliability. To this end, the corresponding high reliability communication protocol in order to ensure the reliable work of the power grid. We compiled the agreement with concise, clear, safe and reliable characteristics. Using C language programming and the strict check, ensure the high reliability of data transmission, for the protection and monitoring of 4 CAN communication structure design
According to the substation protection, automatic control and monitoring requirements, CAN bus communication data according to the nature and the different methods of transmission is divided into five categories.
Periodic transfer: (1) real-time data in accordance with the requirements of substation of real-time data display, generally for 1 second loop transfer, so that after sending the background data processing machine screen display real-time data, ensure the refresh within 3 seconds. This kind of real time data with analog, circuit breaker and the status of the knife, etc. Considering the overall efficiency of communication, still USES the frontend and cyclic query to finish of each interval should be based on the maximum number of nodes and maximum data station to determine the condition of the CAN bus communication rate;
(2) the normal operation of transformer substation data communication with remote control, remote regulating protection setting, and reactive power control commands and radio set number issued, pair, etc., are made by frontend starts the CAN communication, but it takes precedence over cyclical transmission;
(3) the intervals of random information with switch and knife displacement information of the operation, the forecast warning information of the equipment operation, both has the time scale. This kind of information on real time than two kinds, it should be happen nodes quickly to frontend and scheduling;
(4) the accident information refers to the accident, the corresponding equipment interval protections will be immediately accident information and wave record data to the frontend and scheduling, its priorities than random information; Each interval (or nodes) between the protection and control the transmission of information, this information has a transformer mothball protection action information, such as the transformer three tilting jumped for the first time of each side of the mother separated off; Automatic control device of export or blocking information, such as the low voltage side of transformer in hand when the main switch locking side of BZT accordingly; The low voltage side of transformer protection action jump when the main switch, start the corresponding side of BZT; In 35 kV and 10 kV mother separated closed interval of low cycle load shedding exports jump interval corresponding lines; This kind of communication between nodes is the highest priority.
5 the CAN communication principle and its practical application
For a very convenient and reliable interface. CAN bus belongs to the category of the fieldbus, and according to the concept of field bus, CAN bus system model should be the general. The bus structure is used to network topology. Automation equipment adopts industrial control PC, the control node for each interval of CPU, both ends of the CAN bus has the longitudinal resistance (generally about 120 Ω).
(1) the TIMESTAMP: once a frame data detected after the ID number of the TIMER will be high 8-bit values copy to the corresponding MB of the field;
(2) CODE: send and receive the control section, which can, in accordance with the design of the corresponding values of type to complete sent or received. When the MB to send the position 0 xc, will send data in MB unconditional, if sending is successful, the field values to 0 by 8. So in the process of sending, when send completed, should detect whether the field value of 0 by 8, if is not equal to the value, should be waiting, until all the MB data sent successfully completed, and then allow the next send MB; When the MB is used for receiving, if the field is zero, then the MB is not yet ready, if 0 x40, indicates the MB data area is empty, can receive the next data. If successfully received one MB of data, the corresponding IFLAG will buy 1. Therefore, in the process of receiving, when after receiving a MB data, should detect IFLAG corresponding value, until the corresponding position 1, again to allow the next MB, which can ensure the received data is not lost;
(3) LENGTH: the total number of bytes to send data by, when the MB for sending MB, the field value should be the same as the total number of bytes sent by. When MB do receive, then the field will be the total number of bytes of data sent in automatic put inside the corresponding LENGTH;
(4) ID: MB sent by the corresponding identifier, standard frame ID for 11, extend frame for 29;
(5) 16 TIMERSTAMP: when a MB successfully sent or received, after the completion of this moment to capture the TIMER values will be the field;
(6) DATA BYTE0 - BYTE7: store the sent DATA;
(7) RESERVED: unused.
Step 6 communication
Use the CAN communication module, the following problems should be paid attention to. (1) in the process of sending, perform the following four steps:
1) make all of the BUFFER to use in not ready state, as well as to MBi. Control = 0 x80;
2) the BUFFER to use an ID value, the value is the key, because of low four ID must be 0, here is the data sent in the frame, so the RTR should be 0, so give good ID 0, 0, respectively X0020, 0 x0040 etc.
3) to the use of information within the buffer (MESSAGEBUFFER) into the data to be sent;
4) send to allow a set, and points out that the number of bytes to send data. Mbi. Control = 0 x00c0.
When performing the data sent after step 4, information buffer in bus arbitration, automatically once detect the bus is idle, or the competition to win, the data will be sent out.
(2) in the process of receiving, mainly should perform the following steps:
1) have to use the BUFFER is not ready state, and to MBi. Control = 0;
2) to use the BUFFER assigned an ID value, pay attention to must send BUFFER and the same ID value can receive the corresponding data;
3) receive allows a, pay attention to in the process of receiving, each receives a MB, should be the value of the detection IFLAG see did receive success, if not success should be waiting, until it receives the MB after all of the data, then allow next MB to receive the corresponding data.
7, The conclusion
We apply the above principle of communication, in low-frequency low-voltage automatic load shedding device has been successfully completed in the communication between the lower machine, receiving signal can do accurate. Practice has proved that adopting CAN communication module, interface is simple and convenient programming, it is easy to constitute the user's system. Moreover, it has high real-time performance, flexibility and reliability. Practical application shows that this system has the advantages of simple structure, high reliability, flexible and convenient, in the case of fully considering the site environment, data acquisition and conversion accuracy can reach expected set of indicators, the communication rate can reach the requirement of real-time measurement and control system, completely wrong string is extremely low, running effect is very good. At the same time because of using CAN bus technology, greatly improve the reliability of the system, real-time, the system has high cost performance, simple and convenient installation and maintenance, CAN create a good economic benefits, has broad application prospects.