Retro Isle - Acorn Econet (original) (raw)
The Economical Network
With the launch of their Atom microcomputer in 1981, Acorn first introduced their Econet, or "Economical Network". It was designed with simplicity in mind, being easy to setup both in terms of hardware and software. Econet became very popular with schools and colleges at the time, with the influx of BBC Micros and Archimedes computers. The only computer released by Acorn that was not Econet-compatible were the Electron and Archimedes A3010.
Acorn's 1980 specification for Econet was:
- Up to 255 stations per Econet
- Econets may be joined together by Gateways
- Connection by 4-wire cable to each station (typical cost in 1980 was 12p per metre)
- Station seperation up to 1 Kilometre
- Data transfer rate up to 210 kilobaud
- Differential signals for high noise-immunity and minimal radiation
- Crash-detect circuitry and collision-arbitration algorithm to minimise the need for retries
- Econet hardware fits inside Atom case; eurocard version for Acorn System 2/3/4
- Econet software in 4K ROM, fits on Atom board
- Econet executed automatically on user hitting BREAK key
The Econet network is totally democratic in that all stations have equal status and, unless specifically prohibited, any station may communicate with any other with recourse to an intermediary. The only unique station is the one that generates the network clock, but this may be an Atom or a larger system. On larger networks, the stations at the extreme ends are terminated, the termination circuitry is on the Econet board.
So How Does it Work?
Econet is a 5-wire bus network. One pair of wires are used for the clock, one pair for data transmission and one wire is a common ground. Signalling was to the RS-422 5V differential standard, with one bit transfer 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, although on later 32-bit machines (notably the A3020 and A4000) the Econet connection was available via five of the pins on the 15-pin D-type Network port, which could also accept MAUs (Media Attachment Units) to allow other network connections to be used with the same socket. This port looks similar to an AUI port, but is not compatible.[1]
Each Econet interface was controlled by a Motorola MC68B54 Advanced Data Link Controller (ADLC) chip.
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.
Connections were established using a four-way handshake. The sender would broadcast.
What Does an Econet Consist of?
Every Econet must consist of at least these:-
- A minimum of 2 x BBC Micros (or other Econet-compatible computer) fitted with Econet interface hardware and NFS/DNFS ROM.
- An Econet Clock - each Econet needs one and only one clock - this regulates transmission of data around the network
- Econet leads connecting all the computers and clock together.
- 2 x terminators at the two ends of your Econet network
In addition, an Econet can also have:-
- One or more print servers
- One or more Econet bridges (used to link networks together)
The End of Econet
Econet started to disappear from new Acorn machines in around 1993, with the advent of the RISC PC machines. Before this, all machines could accept Econet modules, and even adapters were available to support the different Econet ports for the BBC Master.
Econets are now mainly operated by retro-computing enthusiasts. Original hardware is becoming harder and harder to find, with auction sites such as eBay being popular for acquiring Econet hardware such as bridges and FileStores. The 68B54 chip used for communicating with the network is no longer manufactured, though kits for the Model B and Master series are available online.
Acorn Econet Publications
In the table below you can view or download some Econet-related publications from Acorn:-
| Description | Download |
|---|---|
| Acorn Econet brochure (in PDF format) |  |
| The Acorn Computer LAN (in PDF format) | |
| Introducing the Econet (in PDF format) | |
| The Econet System Clock and Terminators (in PDF format) | |
| Introducing the Econet (in PDF format) | |
| Acorn Factsheet 1 - Recent Econet Developments | |
| Acorn Econet brochure from 1986 | |
| Two halves of a circuit diagram for an Econet PCB (right-click and choose Save Target As - previewing often doesn't work!) |   |