ARCnet is a LAN protocol similar to Ethernet or Token Ring. It was the first widely available networking system for microcomputers, and became popular in the 1980s for office automation tasks. It has since gained a following in the embedded systems market. ARCnet was originally developed by Datapoint Corporation in 1977 to enable groups of Datapoint 2200 terminals to talk to a shared 8" floppy disk system, and was eventually standardised as ANSI ARCnet 878.1.
ARCnet uses a bus topology together with a token-passing scheme which mediates access to the bus. For embedded applications, a bus topology is much more convenient, since hubs are not required. Nodes on the network are organised into a logical ring. When nodes are inactive, the token is passed around the network from node to node. A node is not allowed to use the bus unless it has the token. If a node wishes to send data, it waits for the token to appear, adds the data and control information to the token, and modifies it so that other nodes will not attempt to use it. When the destination node has received the data, it generates a new token and passes it to the next node.
This system guarantees access to the bus for all nodes on the network. Each node will receive the token within a predictable time frame. This is important for control or robotic applications where timely responses or coordinated motion are required. An ARCnet controller handles the communication protocol, including token passing, network configuration, error handling and flow control. A new node joins the ring automatically without software intervention. The standard data rate for ARCnet networks is 2.5 Mbps, but speeds can range from 19 kbps to 10 Mbps. The maximum packet size is 508 Bytes.
ARCnet is an ideal real-time networking system, which explains its use in the embedded systems and process control markets. ARCnet was originally deployed using coaxial cable but now supports twisted-pair and fibre. ARCnet is frequently found in applications such as industrial control, building automation, transportation, robotics and gaming. Like Ethernet, ARCnet is a data-link layer technology.
This article was first published on the TechnologyUK.net website in January 2009.