When it comes to the design of a system with distributed intelligence, i.e. a
networked multi-processor system, the first question that a design engineer
faces is the choice of an appropriate networking technology that best meets the
design specific requirements. CAN (Controller Area Network) and Ethernet are
both very popular choices, especially when a certain level of
vendor-independence, i.e. the avoidance of manufacturer-specific technologies,
is an important aspect.
The main questions, however, are: What is the main difference between both
technologies? Are there really applications where either technology will do the
job? Is there a certain type of application for either technology? Can both
technologies co-exist in one design and combine their specific strengths? The
answers to these questions cannot be articulated in a firm black and white
manner; there are multiple shades of grey.
Without going into all technical details of CAN and Ethernet (there is good
literature available on both topics) let’s have a somewhat closer look:
The CAN bus is primarily used in embedded systems, and as its name implies, is a
network technology that provides fast communication among microcontrollers up to
real-time requirements. CAN is far superior to conventional serial technologies
such as RS232 in regards to functionality and reliability and yet CAN
implementations are more cost effective. While TCP/IP is designed for the
transport of large data amounts, CAN is designed for real-time requirements and
with its 1 MBit/sec baud rate can easily beat a 100 MBit/sec TCP/IP connection
when it comes to short reaction times, timely error detection, quick error
recovery and error repair.
However, due to a limited data length of 8 bytes per message, the use of CAN is
restricted to small-sized embedded multi-processor applications, where the
transfer of raw process data such as digital/analog I/O information is
sufficient. Higher layer protocols based on CAN, such as CANopen or DeviceNet,
are available in order to allow data transfer of unlimited length and to use CAN
for industrial machinery (including motion control).
Another restriction of CAN, with or without the use of higher layer protocols,
is the limited physical network length. The maximum bus length depends on the
chosen baud rate, i.e. roughly 2000 feet at 125 kBit/sec down to 120 feet at 1
MBit/sec.
The greatest benefit of Ethernet comes with the use of the TCP/IP protocol stack
and the possible connection to a company-intern LAN/WAN or even world-wide
through the Internet. Ethernet plus TCP/IP allow physically larger networks with
far greater information carrying capacity than CAN. Compared to CAN, Ethernet
does not support a non-destructive priority arbitration and message latency or
deterministic behavior cannot be guaranteed. Ethernet is still suitable for
field-bus applications where real-time responses through the network are not
important.
Due to the extensive use in PCs, Ethernet network cards are a low-cost items,
where CAN-PC interface cards are more expensive. While TCP/IP sockets are
provided within the most popular operating systems such as Windows or Linux, the
price for a CANopen or DeviceNet protocol stack can range between a few hundred
Dollars for an API, up to several thousand Dollars for source code. However,
when it comes to implement either CAN/CANopen/DeviceNet or Ethernet-TCP/IP, the
CAN technologies still have a considerable edge in terms of implementation into
embedded devices.
The advantages of both technologies can be combined, for instance, in a system
that requires real-time responses within the network plus the means to provide
service and maintainability over long distances.
As shown in the above picture, an Ethernet-CAN gateway connects a local CAN
network to a company LAN/WAN or even the Internet. The gateway can function
either as a network node with a dynamic or static IP address within a company’s
LAN or even as an E-Mail-Server or Web Server through means of the Internet.
Such a configuration allows send/receive process data, to monitor the CAN
network’s performance for data analysis, error detection, etc. and even upload
program updates.
The EtherCAN device by esd electronics is an Ethernet-CAN gateway with a NET +50
ARM processor that controls the data transfer between CAN and Ethernet. The
Ethernet interface is designed for 10/100 MBit/sec networks. The module supports
the Ethernet communications protocols TCP/IP, Modbus/TCP and ProfiNet. The CAN
interface is based on a SJA1000 CAN controller. The interface is designed
according to ISO11898 with electrical isolation. It allows a data transfer of up
to 1 MBit/sec.
For more information log on to:
http://www.esd-electronics-usa.com/store/p/34-EtherCAN.html