Betting on CAN & CANopen
By Olaf Pfeiffer
There is a good chance that CAN - Controller Area Network (also known as CANbus or CAN bus) will soon come to an embedded system near you. Although being around for 15 years, CAN did not yet reach its peak in regards to market penetration and still has an impressive growth rate. The CiA (CAN in Automation international users and manufacturers association) collects and publishes yearly sales figures and forecasts from the 20ish chip manufacturers producing microcontrollers with on-chip CAN interfaces and CAN transceivers. The numbers for 2001 were especially impressive, as 2001 was not a great year for growth in the overall semiconductor industry. Nevertheless, the sales of CAN nodes increased by about 70% to a total of more than 200 million nodes (sold in 2001) and the conservative estimate for the upcoming years is a continuous growth rate of at least 30%. For the US, this rate is actually estimated to be much higher, as in the US CAN currently does not have such a high market penetration as it has in Europe.
Traditional Market Segments
Traditionally the biggest market segment for CAN nodes is automotive. Almost every passenger car manufactured in Europe has at least one CAN node - and drawing boards with future vehicles show 4 or more CAN networks operating in parallel in a vehicle to increase overall bandwidth and allow for up to 100 nodes to communicate via CAN.
The other big market segment using CAN is industrial control. Higher-layer protocols such as DeviceNet and CANopen bring CAN to the system integration level where system integrators can combine off-the-shelf components in a plug-and-play integration process. Typical off-the-shelf components include a variety of industrial I/O modules, including encoders, drives, pneumatic and hydraulic sensors and actuators. Both user organizations maintaining the higher-layer protocols also offer product catalogs online that list available off-the-shelf products. For CANopen products go to the product web pages of the CiA at www.can-cia.org and for DeviceNet products go to the products web pages of the ODVA - the Open DeviceNet Association - at www.odva.org.
Main Benefits of CAN
One of the more important features for industrial control implementations is "real-time". The real-time behavior of CAN compares reasonably well to other networks. In some applications high-priority messages are transmitted within a millisecond - all depending on configuration and busloads. To further enhance the suitability of CAN for applications with hard real-time requirements, TTCAN was recently specified. Time-Triggered CAN is an enhancement to the message triggering mechanisms of CAN and allows reserving a certain bandwidth for specified messages. Simplified, messages may only be sent within specified time slots after the occurrence of a synchronize message. This allows specifying guaranteed time slots for different types of messages.
Today, the biggest advantages of CAN compared to other networks are the costs and the price/performance ratio. With CAN being implemented in standard microcontrollers from about 20 different chip manufacturers, the hardware costs are not much more than those caused by a regular serial line. However, the performance includes a secure implementation of a data link layer that ensures message transfers with a very strong CRC checksum and arbitrates by message priority (on collision, high priority messages get access to the bus).
For applications where the price per node is not as critical as in high-volume applications, the main advantages come from the higher-layer protocols and the availability of off-the-shelf control and I/O components. With these components it becomes possible to build complex control applications without the requirement to develop all nodes of the network from scratch.
In the past, one "knock-out" criterion against CAN was often the maximum speed/bandwidth versus maximum distance ratio. The maximum bus speed of 1Mbit allows for a data bandwidth of about 500kbit at a maximum bus length of about 100 feet. However speed and bandwidth decrease for longer network distances (over the thumb rule: to double the distance, cut the speed in half). Today, this criterion is weakened by the fact that more and more chip manufacturers produce microcontrollers with multiple CAN controller on-chip. On one hand this allows to multiply the CAN networks handled by a single node (and thus multiplies bandwidth). On the other hand, such chips can also be used to implement efficient bridges or routers that can be used to build larger CAN networks by combining several segments.
Recent Market Segments
Especially the higher-layer CAN protocol CANopen is responsible for CAN's acceptance in a variety of newer applications. This includes many medical applications such as computer tomographs or ER equipment that gets interconnected. A further extension to CANopen made it suitable for safety-relevant applications. The solution earned EU approval so that it can now be used in maritime control and other safety-relevant applications. And in the US, CANopen was recently standardized for forklifts and as communication channel between electric service vehicles and the charger station.
CANopen is also especially suited for embedded machine control. Whenever a machine uses multiple low to medium performance microcontrollers that need to exchange information internally, CANopen is a logical candidate. The biggest advantage of CANopen over many other higher-layer CAN protocols is, that CANopen is "open" enough to be customizable. For each application features can be implemented, omitted or optimized/customized. Additional, application specific communication could be added that does not interfere with existing CANopen based communication. This high-level of customization allows designers to cherry-pick the features needed for an application.
Summary and Outlook
If you are not yet familiar with CAN, we highly recommend that you visit one of the frequent events to get a better technical understanding on how CAN and CANopen actually work. There are CAN application examples for every market segment where embedded microcontrollers are used. Some of the applications not yet mentioned include avionics, home appliances, building automation and autonomously controlled vehicles (driving, swimming, diving and flying) in both civilian and military applications. The number of applications where CAN is used will further increase over the upcoming years, making it almost impossible for embedded design engineers and system integrators to not get confronted by it sooner or later.