ARCNET ̶ An Embedded Real-Time Network
ARCNET, once quite popular in office automation, has reinvented itself into an embedded networking technology that is frequently found in applications such as industrial control, building automation, transportation, robotics and gaming. Extremely popular in Japan with continuing popularity in America and Europe, ARCNET is now making inroads into China with some of the top China universities incorporating ARCNET into its projects.
Over 22 Million Nodes
Like Ethernet and Controller Area Network (CAN), ARCNET is a data-link layer technology with no defined application layer. Designers write their own application layer to meet their particular needs and frequently do not advertise the fact that ARCNET is being used in their product. ARCNET receives no name recognition, but is frequently the network of choice in embedded applications. It is hidden from the user, but with over 22 million ARCNET nodes sold gives credibility that ARCNET is indeed popular.
Originally introduced at about the same time as Ethernet, ARCNET incorporates a token-passing protocol where media access is determined by the station with the token. When a station receives the token, it can either initiate a transmission to another station or it must pass the token to its logical neighbor. All stations are considered peers and no one station can consume all the bandwidth since only one packet can be sent each token pass. This scheme avoids collisions and gives ARCNET its greatest advantage in real-time applications ̶ it is deterministic! By being deterministic, the designer can accurately predict the time it takes for a particular station to gain access to the network and send a message. This is of particular importance for control or robotic applications where timely responses or coordinated motion are needed.
Large Packet Size
How does ARCNET compare to CAN? CAN was originally designed for the automotive electronics market and has experienced success as a device level network when used in conjunction with higher layer protocols such as DeviceNet and CANopen. ARCNET is best considered a controller level network because of its higher performance over CAN. CAN communication is limited to 1 Mbps and it can only send eight data bytes per frame. ARCNET chips can transmit up to 10 Mbps and data packets can be up to 507 bytes in length. Not only is ARCNET faster than CAN, ARCNET can send more data per transmission.
Variable Data Rates Up to 10 Mbps and Bus Topology
Another advantage of using ARCNET is that it supports multiple physical layers. The original physical layer was a dipulse transceiver which was optimized for 2.5 Mbps. Newer generation transceivers are much smaller in size and can operate up to 10 Mbps. A designer can select popular and lower cost EIA-485 transceivers when distances are relatively short. For higher isolation voltage, a transformer coupled device is available that will also operate up to 10 Mbps. The advantage of these transceivers is that they will operate over a bus topology. One of the complaints regarding Ethernet is the need to operate in a star topology which requires the use of hubs. Not only does the hub require a source of power, the designer needs to find a place to mount the hub and it represents a single source of failure. For embedded applications, bus topology is much more convenient since hubs are not required.
Reliable Message Delivery
Another problem with Ethernet is the requirement to add a transport layer protocol. Of course, the most popular transport layer protocol is TCP but the code size for a TCP/IP stack is on the order of 50 kB. Small microcontrollers either lack the memory capacity for this stack or the processing power to execute both the stack and the application. Embedded applications do not require a full seven-layer communication model. An application layer, data-link layer and physical layer are all that is needed. Industrial automation protocols such as DeviceNet and CANopen are prime examples. Their data-link layer is CAN and transport functionality, such as, guaranteed message delivery is handled at the application layer. The same can be done with ARCNET. In fact ARCNET controllers have some built-in transport layer functionality. All messages are appended with a CRC-16 frame check sequence and if the frame was received successfully at the destination station, an acknowledgement is automatically sent back to the originating station without any software intervention. This feature in ARCNET simplifies software development and is absent in both CAN and Ethernet.
Simple Microprocessor Bus Interface
ARCNET has other unique features that facilitate embedded design development. The COM20020 family of ARCNET controllers were designed for a simple eight-bit bus microprocessor interface. The complete communication protocol is handled by the ARCNET controller. This includes token passing, network configuration and error handling. Unlike Ethernet, ARCNET has built-in flow control. A message will not be sent to a destination node without the availability of a receive buffer. This further off-loads the software requirements and ensures that messages need not be resent because of receiver errors. ARCNET controllers are competitively priced and there are no licenses or expensive development platforms that must be purchased. Only standard embedded design tools are required to develop a system.
Included in the BACnet® Standard
It is not only robotics where ARCNET is used. ARCNET is one of the approved data links for the BACnet standard. BACnet ̶ Building Automation and Control Network ̶ was developed by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) and has now become an ISO standard and has become increasingly popular around the world.
Simple to Use
If you ask a designer why they use ARCNET, they will usually say it is simple to use, low-cost, high performing, and extremely rugged ̶ it is bullet-proof! Newer ARCNET controller chips have an extremely small footprint making it ideal for embedded applications. Unlike TCP/IP/Ethernet networks with complex 48-bit addressing and assignment, ARCNET nodes can only number 255 and can be set by a simple 8-position DIP switch.
The opportunities for using ARCNET are enormous. From Pachinko machines and high-speed trains in Japan to process automation and motor drive controllers in Europe. ARCNET continues to perform in non-stop applications.
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