Definition of CAN bus and RS485 bus
Can is the abbreviation of controller area network (can). It was developed by Bosch Company in Germany, which is famous for developing and producing automotive electronic products, and finally became an international standard (iso11898). It is one of the most widely used Fieldbus in the world. In North America and Western Europe, can bus protocol has become the standard bus for automotive computer control system and embedded industrial control LAN, and has J1939 protocol designed for large trucks and heavy machinery vehicles with can as the underlying protocol. In recent years, its high reliability and good error detection ability have attracted much attention. It is widely used in automotive computer control system and industrial environment with bad ambient temperature, strong electromagnetic radiation and large vibration.
RS-485 adopts half duplex working mode and supports multipoint data communication. RS-485 bus network topology generally adopts the bus structure with terminal matching. That is, a bus is used to connect each node in series, which does not support ring or star network. RS-485 adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition, the bus transceiver has high sensitivity and can detect voltages as low as 200mV, so the transmission signal can be recovered from kilometers away. Some RS-485 transceivers modify the input impedance to allow up to 8 times more nodes to be connected to the same bus. The most common application of RS-485 is the communication between programmable logic controllers in industrial environment.
Application overview of CAN bus and RS485 bus
In the past, the communication between PC and intelligent equipment mostly relied on RS232, RS485, Ethernet and other methods, which mainly depended on the interface specification of the equipment. However, RS232 and RS485 can only represent the physical media layer and link layer of communication. If you want to realize the two-way access of data, you must write your own communication application program. However, most of these programs can not meet the ISO / OSI specification and can only realize a single function. They are suitable for a single device type, and the program does not have universality.
In the equipment network connected by RS232 or RS485 devices, if the number of devices exceeds 2, RS485 must be used as the communication medium. In order to exchange information between devices in RS485 network, it can only be realized through “master” device transfer. This master device is usually PC, and only one master device is allowed in this equipment network, and the rest are slave devices. Fieldbus technology is based on ISO / OSI model and has a complete software support system, which can solve the problems of bus control, conflict detection, link maintenance and so on. Fieldbus devices are automatically networked, without master / slave devices, or multiple masters are allowed. At the same level, the products of different manufacturers can be exchanged, and the equipment has interoperability.
There are many bus formats now. What are the characteristics of can compared with other buses? First of all, let’s compare the familiar 485 bus. 485 bus is only a level standard, not a new protocol, similar to 232. Of course, it’s not very appropriate, but it helps you understand.
Can bus PK RS485 bus
Can (controller area network) belongs to the field bus. It is a serial communication network that effectively supports distributed control or real-time control. Compared with the current RS-485 distributed control system based on R line, the distributed control system based on CAN bus has obvious advantages in the following aspects:
1) The CAN controller works in the multi master mode. Each node in the network can compete to send data to the bus by using the bit by bit arbitration mode of lossless structure according to the bus access priority (depending on the message identifier), and the CAN protocol abolishes the station address coding and instead encodes the communication data, which can enable different nodes to receive the same data at the same time, These characteristics make the data communication between the nodes of the network composed of CAN bus strong in real time, and easy to form a redundant structure, so as to improve the reliability and flexibility of the system. RS-485 can only form a master-slave structure system, and the communication mode can only be carried out in the way of master station polling. The real-time performance and reliability of the system are poor.
2) The CAN bus is connected to the physical bus through the two output terminals canh and canl of the CAN controller interface chip 82C250. The state of the canh terminal can only be high-level or suspended state, and the canl terminal can only be low-level or suspended state. This ensures that the bus will not be short circuited and damage some nodes when the system has errors and multiple nodes send data to the bus at the same time, as in RS-485 network. Moreover, the CAN node has the function of automatically closing the output in case of serious errors, so that the operation of other nodes on the bus will not be affected, so as to ensure that it will not appear in the network. Due to the problems of individual nodes, the bus is in a “deadlock” state.
3) Can has perfect communication protocol, which can be realized by CAN controller chip and its interface chip, which greatly reduces the difficulty of system development and shortens the development cycle, which can not be compared with RS-485 with only electrical protocol.
Characteristic comparison between RS-485 and can bus