ARCNET

Attached Resource Computer NETwork (ARCNET or ARCnet) is a communications protocol for local area networks. ARCNET was the first widely available networking system for microcomputers and it became popular in the 1980s for office automation tasks. It was later applied to embedded systems where certain features of the protocol are especially useful.

ARCNET was developed by principal development engineer John Murphy, at Datapoint Corporation in 1976 under Victor Poor, and announced in 1977. It was originally developed to connect groups of their Datapoint 2200 terminals to talk to a shared 8" floppy disk system. It was the first loosely coupled LAN-based clustering system, making no assumptions about the type of computers that would be connected. This was in contrast to contemporary larger and more expensive computer systems such as DECnet or IBM's SNA, where a homogeneous group of similar or proprietary computers were connected as a cluster.

The token-passing bus protocol of that I/O device-sharing network was subsequently applied to allowing processing nodes to communicate with each other for file-serving and computing scalability purposes. An application could be developed in DATABUS, Datapoint's proprietary COBOL-like language, and deployed on a single computer with dumb terminals. When the number of users outgrew the capacity of the original computer, additional 'compute' resource computers could be attached via ARCNET to run the same applications and access the same data. If more storage was needed, additional disk resource computers could also be attached. This incremental approach broke new ground and by the end of the 1970s (before the first IBM PC was announced in 1981), over ten thousand ARCNET LAN installations were in commercial use around the world while Datapoint had become a Fortune 500 company. As microcomputers took over the industry, well-proven and reliable ARCNET was also offered as an inexpensive LAN for these machines.

ARCNET remained proprietary until the early-to-mid 1980s. This did not cause concern at the time, as most network architectures were proprietary. The move to non-proprietary, open systems began as a response to the dominance of International Business Machines (IBM) and its Systems Network Architecture (SNA). In 1979, the Open Systems Interconnection Reference Model (OSI model) was published. Then, in 1980, Digital, Intel and Xerox (the DIX consortium) published an open standard for Ethernet that was soon adopted as the basis of standardization by the IEEE and the ISO. IBM responded by proposing Token Ring as an alternative to Ethernet but kept such tight control over standardization that competitors were wary of using it. ARCNET was less expensive than either of these, was more reliable, more flexible and, by the late 1980s, had a market share about equal to that of Ethernet. ^{[citation needed]} Tandy/Radio Shack offered ARCNET as an application and file sharing medium for their TRS-80 Model II, Model 12, Model 16, Tandy 6000, Tandy 2000, Tandy 1000 and Tandy 1200 computer models. There were also hooks in the Model 4P's ROM to boot from an ARCNET network.

Ethernet became much more attractive when it moved from co-axial cable to twisted pair and an "interconnected stars" cabling topology based on active hubs. Easier cabling, combined with the greater raw speed of Ethernet (10 Mbit/s versus 2.5 Mbit/s for ARCnet) helped to increase Ethernet's demand. As more companies entered the market, the price of Ethernet started to fall while ARCNET and Token Ring volumes tapered off.

In response to greater bandwidth needs, and the challenge of Ethernet, a new standard called ARCnet Plus was developed by Datapoint and introduced in 1992. ARCnet Plus ran at 20 Mbit/s and was backward-compatible with original ARCnet equipment. However, by the time ARCnet Plus products were ready for the market, Ethernet had captured the majority of the network market and there was little incentive for users to move back to ARCnet. As a result, very few ARCnet Plus products were ever produced. Those that were built, mainly by Datapoint, were expensive and hard to find.

ARCNET was eventually standardized as ANSI ARCNET 878.1. It appears this was when the name changed from ARCnet to ARCNET. Other companies entered the market, notably Standard Microsystems who produced systems based on a single VLSI chip, originally developed as custom LSI for Datapoint, but later made available by Standard Microsystems to other customers. Datapoint eventually found itself in financial trouble and moved into video conferencing then and later to custom programming in the embedded market.

Even though ARCNET is now rarely used for new general networks, the diminishing installed base still requires support and it retains a niche in industrial control.