On today’s automation solutions, the number of electrical components continues to grow. The cabling of these electrical components requires a great effort, involving huge costs at the project planning and complex wiring schemes. For that reason, so-called “fieldbus systems” were developed, that transfer the data with help of only one single bus cable, which passes all field devices. Thus, the wiring effort and the weight could be drastically reduced. The “CAN-Bus” is probably the most famous fieldbus system, which mainly established a reputation in the sector of automotive engineering. The most important information among this topic is summarized in this article.
CAN-Bus – equal control units
The CAN-Bus was invented as an automotive-bus-system by the German company Robert Bosch GmbH to the requests of the automotive manufacturers Daimler and BMW. In cooperation with the company Intel, the first CAN-Chips were presented in 1987. For the first time, the CAN-Bus was used at the serial production of the S-class of Mercedes in 1992. The purpose of this development was to reduce the cabling drastically to make the vehicle lighter.
If you are using CAN-Bus, the multi-master principle applies. That means that all control units have equal rights and are authorized to transmit their information whenever they want to. Therefore it is possible that data collisions occur, when data is transmitted simultaneously. Thus, the recipient is not able to read the messages anymore. By the use of the Carrier Sense Multiple Access / Collision Resolution, a prioritization of the data transmission is done. This method solves the problem of colliding data but thereby the CAN-Bus is not real-time capable anymore.
Topology and functionality
The cabling of field devices at the CAN-Bus is mostly provided in a line topology. The information is sent from field device to field device and every single device reads the necessary information and passes its own data along to the following devices. The use of branch lines or a star-shaped structure (e.g. central locking system in a car) is also possible, but comes with some restrictions.
Data transfer through the CAN-Bus is serial. That means the data is sent consecutively. Therefore a fixed process is used, so that every single field device is able to recognize which data of the bit string passing through, is interesting. For this process, standardized frames with the same structure are used. Besides the recipient of the message, these frames also contain the request to return data or error codes. In the case that five identical bits are sent in a row, the CAN-protocol recognizes an “end-of-frame”. However, it is possible that there are data sequences that must transmit five bits with an equal level. That is when the “bit stuffing” goes into action. After five identical bits, one bit with an inverse level is inserted and therefore illustrates that there is no “end-of-frame” yet.
The CAN-Bus is distinguished into a high-speed and a low-speed-CAN. The difference between these two types is the data transmission rate and the voltage. A CAN-Bus normally is realized with two or three wires: CAN_HIGH, CAN_LOW and optional CAN_GND. Using the high-speed-CAN, the voltage level, when not in use (recessive), is 2,5V. If a dominating bit is transmitted, the voltage of CAN_HIGH rises up to 3,5V while the voltage of CAN_LOW falls down to 1,5V. When using the low-speed-version, the recessive voltage of the CAN-Bus is 0V (CAN_LOW) and 5V (CAN_HIGH). When transferring dominating bits, the voltage value turns into 1,4V (CAN_LOW) and 3,6V (CAN_HIGH). If one of the two lines doesn’t work, it is possible to evaluate voltage against ground Basically when using the low-speed version, a single-wire-system of the CAN-Bus would be enough, but for safety reasons the double-wired-system is used to carry the business on if the first wire fails.
Due to its roots in the automotive sector, the CAN-Bus became the mostly used bus system. But it is not able to fully satisfy the demands of modern automation. Especially real-time capability is necessary if you want to control several electric motors or hydraulic cylinders simultaneously. For this reason, bus systems of the second generation, which are usually upgrades of the Ethernet standard, got great advantages. Nevertheless CAN-Bus will still be used in many areas of future automation. Hence the Generic Control Box of the Robot Makers GmbH is offering an interface to this very important field bus.