By Andreas HeldweinLONG ESTABLISHED
and widely used in automotive applications, the Controller Area Network (CAN) protocol is steadily growing in industrial settings as a means to connect servos, sensors, controllers, and other machine control and automation devices. As a result, engineers are seeking to improve CANís performance and implementation in industrial networks, which is fueling demand for integrated industrial CAN semiconductor devices.
While most network specifiers in our reader industries wonít work at the semiconductor device level, it will be useful to have a look at chip-level architectures to gain some understanding of how those design decisions effect network performance.
CAN originally was developed in the 1980s to provide multiplexed serial bus communications in vehicles. Its aim was to improve functionality and simplify, reduce, and lighten complex data wiring in automotive electronics.
Now designated as international standard ISO 11898, CAN is based on a mechanism that identifies transmitted messages purely by content rather than by the node address of the transmitting or receiving device, which is the case with most other bus systems. Consequently, all nodes on a network receive and evaluate messages based on relevance and priority, and act or discard them as needed. Messages can be targeted to specific nodes or sent to many nodes. This content-orientated addressing provides more system and configuration flexibility, allowing CAN nodes to be added quickly and easily to existing networks without adding hardware or modifying software.
CANís flexibility, ease of use, and reliability, combined with its cost-effective, wire-reducing capabilities, have increased its industrial popularity in recent years. AMI Semiconductor
estimates that up to 20% of worldwide CAN implementations presently occur in the industrial sector. Applications using CAN include factory automation, warehousing, and building control. CAN networks also are used to connect systems, such as textile machines, printers, conveyors, ovens and refrigeration units.
Likewise, two higher-layer, CAN-based protocols, CANopen and DeviceNet, often are used in industrial automation. Historically, DeviceNet is employed more in North America. CANopen is successful in European automation and motion control networks, mostly in public transport, building automation, maritime electronics, and medical equipment applications, according to CAN in Automation (CiA), an international user and manufacturer association that develops and standardizes CANopen-based device and application profiles.CABLES AND PROTECTION
Naturally, there are some key differences between automotive and industrial environments, which industrial designers need to consider when selecting and implementing CAN technology. Many non-safety-related automotive applications typically use low-data-rate CAN specifications of less than 500 kbps, rather than the faster 1 Mbps needed for many industrial designs. Also, CAN hardware used on the plant floor obviously must handle longer cable lengths than wiring in vehicles, though data rates slow as distance increases.Cable Lengths Affect CAN
Run Length (meters)
Signal Rate (kbps)
In addition, these longer cables usually run through harsh, electrically noisy environments, which make protection against electromagnetic interference especially crucial. It is in these areas that semiconductor manufacturers are concentrating their industrial CAN technology developments.
The physical layer is one of the most critical components of a CAN network because itís essential to delivering robust performance. In industrial applications, the physical layer typically will be the high-speed version defined by the ISO 11898-2 standard, which runs at up to 1 Mbps. This standard defines medium access unit (MAU) functions and some medium-dependant interface (MDI) features. To implement physical layer functions, CAN transceiver circuitry that combines connection to the two-wire bus with physical layer capabilities is required.
In recent years, this circuitry has been integrated into single ICs to minimize component count and simplify manufacture. Several major companies manufacture these devices, including Texas Instruments, Infinion, Phillips, Microchip, and AMIS. Figure 1
below shows a block diagram of the AMIS-30660 conformance-tested, Vd1.1-certified, ISO 11898-2-compliant, high-speed CAN transceiver device. Supplied in an SO-8 package, this ICís single-chip solution provides differential transmit capability to the physical CAN bus and differential receive capability to the CAN protocol controller, and provides high levels of protection against the transients found in industrial environments.A block diagram of this Vd1.1-certified, ISO 11898-2-compliant, high-speed CAN transceiver device shows how an IC single-chip solution can provide differential transmit capability to the physical CAN bus and differential receive capability to the CAN protocol controller.
These newer CAN devices offer advantages over conventional off-the-shelf transceiver ICs, especially in protecting against electromagnetic susceptibility (EMS), electromagnetic emissions (EME) and electrostatic discharge (ESD). High-frequency energy exposures due to long cables can hinder CAN transceivers or halt their bus communications completely in severe cases. As a differential bus, CAN already is low-noise with respect to EME. These emissions are generated whenever digital logic is switched from one state to another, such as when signals are sent via I/O pins or when drivers are turned on or off.