RS485 vs CAN

This question usually arises during the initial evaluation of communication technologies in any project with distributed control requirements. This article pretends to describe the differences between these two technologies and finally offer a way of deciding which of them is the best for our application, bearing in mind the key requirements of our project: lead time, budget, platform and development skills.

RS485 was created in 1983 as a way of providing multidrop communication capabilities for devices with at least one UART. RS485 is often called the "multiplexed" RS232 - any of the devices connected to the bus can talk to any other in half-duplex mode. Half-duplex means that the communication is bidirectional but only one device can transmit at a time (one transmits, the other listen to it, and so on). But RS485 is not a protocol, it's just the physical path and the basic rules over which any device can communicate. RS485 provides a way of transmitting serial messages over the multidrop bus but the contents of these messages are totally user-defined. The structure of the communication frame, the time that a message must be transmitted at, the way of addressing devices on the bus, the method for avoiding data collisions, etc are some of the steps that a designer must cover when defining a protocol over RS485.

CAN (Controller Area Network) was created in the 80's by Robert Bosch GmbH and it initially targeted the automotive industry. CAN, in contrast to RS485 not only provides a physical media for communicating but also defines the necessary mechanics for addressing packets and avoiding data collisions. CAN specifies the structure of the data frame - the position and number of bytes for the address the data and the control bytes. Everything follows a precise structure and a timing in order to guarantee quality of transmission, delivery of every transmitted packet, speed and also avoid corruption of data. CAN is thus a very secure technology, and because of that it's currently been used in critical environments as vehicles, planes, vessels and the industry in general.

Implementing CAN from scratch is not necessary as there is nowadays an important amount of manufacturers selling CAN controllers and microcontrollers with the whole CAN stack included. As result, CAN is often preferred because it provides a simple way of designing true multimaster systems without having to define a protocol from scratch. On the other hand, RS485 is typically used in master-slave environments, where a data collision detector is not necessary. The cost in components is also lower in the RS485 case as most microcontrollers have an UART port, even the smallest ones. On the other hand, CAN usually needs more expensive microcontrollers with larger memory and an integrated CAN controller or a SPI port for driving an external CAN controller. This makes CAN a bit overkilled for small distributed sensors even if it can't be considered an expensive solution.

The following table tries to summarize the most common features of both technologies:

Feature	RS485	CAN
Necessary microcontroller interface	UART	CAN controller or SPI
Native system for detecting data collision	No, it must be implemented in software if necessary	Yes, CSMA/CD
Maximum communication speed	10 Mbit/s	1 Mbit/s
Maximum bus length	1200 m (at 100 kbit/s)	500 m (at 125 kbit/s)
Layers in the ISO model	Physical layer	Physical layer and data link layer
Maximum amount of data into a single frame	Unlimited, it depends on your application	8 bytes
Component costs	Very low	Low-medium
Development time	medium/high	low/medium
Typical use	Master-slave applications	Multimaster applications
Examples of popular protocols	Modbus RS485, Profibus	CanOpen, Devicenet, J1939

Does this mean that RS485 is only suitable for master-slave protocols? Not necessarily. A number of vendors have implemented their own proprietary multimaster protocols based on RS485. The way they detect collisions and ensure data integrity is not known but the solution itself is indeed possible. Some open multimaster protocols use control bytes for reserving/releasing the bus and even detecting collisions when at least two address fields get overlapped. Another possible solution is to use of some kind of synchrony between nodes.

Other sources of information:

http://www.can-cia.org
http://en.wikipedia.org/wiki/Controller_Area_Network
http://en.wikipedia.org/wiki/RS-485