Econet

Econet was Acorn Computers's low-cost local area network system, intended for use by schools and small businesses. Econet is rumoured to be an abbreviation of Economy Network, but Acorn were always careful to stress the Greek root, oikos, meaning "house".

Implementation history

Econet was first developed for the Acorn Atom and Acorn System 2/3/4 computers in 1981. Also in that year the BBC Microcomputer was released, initially with provision for floppy disc and Econet interfaces but without the necessary hardware fitted, intended to be supplied as future upgrades.

In 1982, the Tasmania Department of Education requested a tender for the supply of personal computers to their schools. Earlier that year Barson Computers, Acorn's Australian computer distributor, had released the BBC Microcomputer with floppy disc storage as part of a bundle. Acorn's Hermann Hauser and Chris Curry agreed to allow it to be also offered with Econet fitted, as they had previously done with the disc interface. As previously with the Disc Filing System, they stipulated that Barson would need to adapt the network filing system from the System 2 without assistance from Acorn. Barson's engineers applied a few modifications to fix bugs on the early BBC Micro motherboards, which were adopted by Acorn in later releases. With both floppy disc and networking available, the BBC Micro was approved for use in schools by all state and territory education authorities in Australia and New Zealand, and quickly overtook the Apple II as the computer of choice in private schools.

With no other supporting documentation available, the head of Barson's Acorn division, Rob Napier, published Networking with the BBC Microcomputer,^[1] the first reference documentation for Econet.

Econet was officially released for the BBC Micro in the UK in 1984, and it later became popular as a networking system for the Acorn Archimedes. Econet was eventually supported on all post-Atom Acorn machines, apart from the Electron (except in Australia and New Zealand where Barson Computers built their own Econet daughter board), the A3010, and the eventually-cancelled Phoebe 2100.

Econet was supported by Acorn MOS, RISC OS and RISC iX.^[2] Acorn once received an offer from Commodore International to license the technology, which it refused.^[3]

An "Ecolink" ISA interface card for IBM-compatible PCs was available.^[4] It used Microsoft's MS-NET Redirector for MS-DOS to provide file and printer sharing via the NET USE command.

Subsequent development

With the falling prices and widespread adoption of IP networking in the early 1990s, Acorn Universal Networking (AUN), an implementation of Econet protocols and addressing over TCP/IP, was developed to provide legacy support for Econet on Ethernet-connected machines.^[5]

The Econet protocol and AUN were also supported by the Linux kernel.

Supported systems

Econet was supported by a large number of different computer and server systems, produced both by Acorn and by other companies. As well as Acorn's MOS and RISC OS these also used other operating systems such as CP/M, DR-DOS, Unix or Microsoft Windows.

The Econet API includes an Econet_MachinePeek command, which can be used by software to determine if a machine is present on the network and its hardware platform. The machine type numbers which can be returned by that command^[6][7] are a useful indication of the range of hardware that offered Econet as their primary networking function or as an option:

The manual^[7] includes an assembly language program to report a machine type, software version and release numbers.

An update to the list in volume 5A of the PRM^[8] lists the following additions to the table above:

Physical and Data Link layers

Econet DIN-5 Pinout

1	Data (+ve)
2	Ground
3	Clock (+ve)
4	Data (-ve)
5	Clock (-ve)

Econet is a 5-wire bus network. One pair of wires is used for the clock, one pair for data, and one wire as a common ground. Signalling used the RS-422 5-volt differential standard, with one bit transferred per clock cycle. Unshielded cable was used for short lengths, and shielded cable for longer networks. The cable was terminated at each end to prevent reflections and to guarantee high logic levels when the bus was undriven.

The original connectors were five-pin circular 180° DIN types. On later 32-bit machines (notably the A3020 and A4000), the Econet connection was made via five of the pins on their 15-pin D-type Network port, which could also accept MAUs (Media Attachment Units) to allow other types of network to be connected via the same socket. This port looks similar to an AUI port, but is not compatible.^[9]

Each Econet interface was controlled by a Motorola MC68B54 Advanced Data Link Controller (ADLC) chip, which handled electrical transmission/reception, frame checksumming and collision detection.^[10]

Each network segment had a maximum length of 500 meters, and could have up to 254 devices ("stations"). Machines and appliances such as filestores and bridges were configured with unique station numbers using jumpers or CMOS RAM settings. Network bridges, housed in a standard "BBC Cheese Wedge" box, were available for building larger networks; up to 127 segments could be bridged together.

The clock signal was generated either by a stand-alone clock box, by a BBC Microcomputer with a modified Issue 4 mainboard, or by a Filestore fileserver. Only one clock generator could be used on each network. While the network was originally specified to run at 210 kHz, practical clock frequencies could range from about 40 kHz to around 800 kHz; the presence of older machines on the network or the capacitance of a long network cable would reduce the maximum data rate reliably available.

Network and Transport layers

Econet used a connectionless transmission model, similar to the current UDP, with no checksumming or error correction at this layer. Each packet had a four byte header consisting of:

1. the destination station number
2. the destination network number
3. the source station number
4. the source network number

A single data transmission consisted of four frames, each with a header as above:

1. The sending station sends a scout packet with a port number and a flag byte
2. The addressed receiving station returns a scout acknowledge to the sender
3. The sending station sends the data
4. The receiving station finishes with a final acknowledge, identical to the scout acknowledge

There was provision for broadcast transmissions, a single frame sent with its destination station and network numbers set to 255. There was also provision for promiscuous mode reception, termed wild receive in the PRM, requested by listening for station and network numbers both being zero.

Technical details of packets and frames, the Econet API, and worked examples in ARM assembler and BBC BASIC are given in the RISC OS Programmer's Reference Manual.^[11]

Network services

At the time and in the markets for which Econet was developed, the main purpose of computer networking was to provide local area shared access to expensive hardware such disc storage and printers. Acorn provided software for the BBC Micro to implement a file server, and optionally a printer server also. The original file server was very basic, essentially allowing limited access to a floppy disc over the network. The server software was further developed over many years, and Acorn and other manufacturers also produced dedicated Econet servers based on various technologies. So the servers available fell into roughly three categories:^[12]

• The Acorn Level 1 through Level 4, running on a standard computer (BBC, Master or Archimedes) and providing simple file and print services.
• The dedicated Acorn FileStore units, running on dedicated hardware with higher capacity and more facilities.
• Third party units (notably from SJ Research), again running on dedicated hardware and with their own implementations of the server software. These were compatible with the Acorn implementations, but with additional enhancements.

The machine type numbers listed in the "Supported systems" section above are an indication of the range of hardware that was available or planned.

Additional services could be implemented, using the network API provided. Short utilities such as network chat programs were often published in magazines or distributed by sharing among users; these made use of the Econet protocols to work alongside the basic file and print services. Larger software packages (some of them commercial) were available that provided services such as Teletext and modem drivers.^[13]

Comparison with modern systems

While Econet can be considered unique amongst network systems and specific to the Acorn range of computers, it does share many commonalities with modern network file systems and protocols:

• Remote Procedure Call – Almost all network operations were performed via a primitive remote procedure call system, either by passing a command line direct to the file server, or by passing an operating system call parameter block. The logon command *I AM was processed by passing the whole command line and reading back the result code.

• Access Permissions – By the time of the Acorn Level 4 File Server and the SJ Research MDFS systems, Econet file servers had a full user name and password system with public and private attributes. These worked similar to Unix permissions without the group field. Files could be set to be readable and/or writable by everyone, just by the user, or both.
• Subnetting – A basic Econet would be a single network segment, which is usually assumed to be network 0. With the use of one or more bridges, it is possible to have up to 127 Econet segments with up to 254 hosts each, for a maximum of 32,258 possible machines.
• Broadcasting – By using host 255, an Econet host could send broadcast packets to all hosts on the network segment. Later implementations of the client software used this to automatically locate file and printer servers.
• Printer Spooling – Later versions of the Econet printer server software used printer spooling to locally cache print jobs before sending to the remote printer. This ensured whole print jobs were sent to the printer in one go.
• Ports – Because the various protocols (file and printer servers, bridge discovery, and so forth) used defined port numbers,^[14][15][16] it was possible to for additional services such as BroadcastLoader, AppFS, a teletext server, and a range of chat programs and multiplayer games to coexist within the Econet system.

Decline and obsolescence

Econet was developed at a time (starting in the early 1980s) when standalone computers were common even in education and business environments. Computer networks were only just beginning to emerge, and those that existed were often seen as exotic research projects. There were no standards, all of them using custom designs and private protocols.

Econet provided a solution for shared access to expensive hardware, an especially attractive option for Acorn's educational customers. However, due to the relatively slow procurement cycles in that sector, new or substantially improved Acorn designs tended to be released over relatively long intervals, meaning that new designs still needed to interoperate with more limited older ones.

In the wider computing industry, increasing performance and data throughput demands led to the widespread adoption of technologies such as Ethernet and TCP/IP; these were accepted standards that could be supported by any manufacturer. By the early 1990s Ethernet had evolved from its original bus network into a point-to-point hub based system, providing lower cost and easier cabling as well as higher data rates. Econet, with its need to maintain backwards compatibility, was unable to do the same.

At this time Acorn was attempting to market their systems not only to their traditional education sector, but also to higher education (with support for Unix) and small businesses. Even home users may have had not just one computer but maybe more. In this increasingly heterogeneous computing environment, Acorn systems began to acquire support for foreign disc formats, archive files and media files.

With the falling cost of standard Ethernet networking, Econet began to lose its relevance. With the introduction of the RiscPC in 1994, Econet hardware was no longer supported as standard on new Acorn machines. Previous machines had accepted add-on Econet modules via a similar connector to that used in BBC Master computers, but the RiscPC range used a new network slot connector that would not accept the old modules. An Econet podule (the RiscPC's name for a plug-in expansion card) could still be fitted, and subsequently an Econet module was produced for the new network connector.

Econet software was mostly superseded by AUN, though some suppliers were still offering bridging kits to interconnect old and new networks. In turn, AUN was superseded by the Acorn Access+ software^[17] that used native TCP/IP over Ethernet rather than by simulating Econet packets over UDP. This in turn was superseded by native support for standard protocols such as the Network File System (NFS) and SMB/CIFS.

Econet networks are now mainly operated by retrocomputing enthusiasts. The supply of original hardware is limited, with auction sites such as eBay being popular for acquiring Econet hardware such as bridges and FileStores. The 68B54 ADLC used for interfacing to the network is no longer manufactured, although kits for the Model B and Master series are still available.

Support for Econet was removed from the Linux kernel at version 3.5 in 2012.^[18][19]

See also

• List of device bandwidths

External links

• The Econet Enthusiasts Area
• Chris' Acorns
• Econet documentation at 8-bit software
• RISC OS Programmer's Reference Manuals – the latest versions as of May 2014

References