What is Econet?

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What is it?

Econet is a networking system designed by Acorn Computers Ltd. It uses a 6854 ADLC (Advanced Data Link Controller) to implement a serial network across a five-wire cable.
The cable consists of two twisted pairs. One pair for the data signals, the other pair for the clock signal. The final wire is an earth.

The system is flexible. The proper wire is a thick shielded wire consisting of two twisted pairs as described above, however if you are willing to sacrifice speed, you can use standard six-core telecoms cabling.

The network topology operates on the principle of a "backbone". This passes through a number of socket boxes into which 'drop leads' may be plugged. A drop lead is nominally a special Econet lead, however as it is a 5 pin DIN (180°), people often use MIDI patch leads as obtaining a pre-built Econet lead these days is near impossible.

Basic network topology
This diagram shows an MDFS server (on the left) connected to the
backbone of the network, which is split at a clock box, and is
then connected to three generic 'Archimedes' workstations.


What you need

The network should consist of at least four consituent parts:
  1. A file server
    This provides the functionality for users to access data and software in a unified manner.
    While the early Econet setups required a dedicated "printer server", this has been integrated into the FileStore and Level4, as well as being provided as standard on all SJ servers - the most common of which is the workhorse MDFS.
  2. A clock box
    In order to function, the network needs a device to place regular clock pulses on the clock lines of the network cable.
    The usual clocking speed for a network running older equipment is 1µS mark and 4× space (in total 5µS per bit). The FileStore, depressingly, does not appear to work at anything much faster when though its clock can be set to 1µS/1×. This offers a throughput in the order of 11Kb/sec.
    If you do not need BBC/Master or FileStore compatibility, you can easily double this. The SJ bridge and the MDFS will mostly keep up with an Archimedes. You may be able to achieve in excess of 60K/sec by fine-tuning your network.
    The most adjustable clock is the SJ Bridge, followed by the SJ clockbox, both of which permit the timing to be tweaked down to divisions of a microsecond; for example ¾µS mark, 3½µS space.
    Note - however - there is a lot more potential for speed tweaks on a short network in a bedroom than for a system spanning several classrooms and dozens of machines.
  3. A terminator
    The purpose of a terminator is to apply a damping resistance to the network cable to 'suck up' the energy of the transmission and thus prevent signal reflections which could interfere with subsequent data on the network. In general, the faster the network or the longer the cable, the more important the termination becomes.
    This is not an unusual concept. "Cheapernet" (10base2) requires terminators, SCSI cables should be terminated at each end, Econet is no different here.
  4. A "station"
    What point a network if you have nothing to connect it to?
    Pretty much every Acorn machine (with the notable exceptions of the Electron and the A3010) could be fitted with Econet - and I believe third-party expansions were available which offer Econet to both of those systems.
    In the systems produced before the BBC Master, Econet upgrades involved physically soldering the required components onto blank spaces on the motherboard, plus installing a ROM which provided networking services (typically "NFS" for networking services, or "DNFS" for floppy disc/networking).
    Starting with the Master 128, Acorn refined their machines to use a plug-in board which makes life a lot easier. This same board can be fitted into the Master, the FileStore, an A5000, etc. Notable exceptions were the A4, the A4000, and the RiscPC due to the form of the machine.
    An ISA card was produced for PCs, however the driver is apparently non-Windows, and you'd be hard-pushed to find an ISA socket these days.
Using these components, you can put together networks of immense cabability. A simple network can consist of up to 253 stations (0 and 255 are 'call' addresses, 254 is typically the server). By using a device called a bridge, you can link two disparate networks, and transmit data between the two. Suddenly a network addressing couple of dozen machines in one block could be four separate, yet connected, networks across campus, where it is possible for any one machine to connunicate with, potentially hundreds.

The very simplest network is a much cuter affair. The Acorn FileStore contains an internal clock. It is entirely possible to connect a FileStore to a BBC micro (or A3000, etc) back-to-back using a drop lead. With a Y-splitter, you can connect together three drop leads to have a network comprising of a server and two stations. Terminators are not an issue with a network this tiny, and anyway the FileStore runs at a speed somewhere between slug and caterpillar!

Windows share optionsLikewise, you can network RISC OS machines using simply a clockbox and some drop leads to join it all together. Alan Williams has produced a freely available server (awServer) which puts in a good emulation of a FileStore... run this on one of the machines, what more do you need?
There is also the concept of ShareFS which allows directories (or entire discs) to be shared amongst RISC OS machine in an arbitrary fashion (each 'share' is autonymous, there is no 'server' as such). This is similar to the Windows 'Share files and folders' (as shown on the right).


The glory days

You may have read the above text and choked on your tea when you got to the bit about the throughput being 11K/sec upwards, but less than 100K/sec. It is easy to feel that way when 10baseT has been around a while, then 100baseT, then GigaBit, and it is fairly common for a sustained 16Mb/sec broadband connection right into your computer (unless you happen to live in the UK in which case the maximum usual offer is 8Mb, of which you may only ever achieve half that except during unsociable hours).
The reality of the situation is that Econet was a very flexible, very affordable system to connect BBC micros together in a classroom environment. Each station could be fitted out with Econet for under a hundred pounds, and it cost around a thousand pounds for an E01S FileStore server (Acorn price index, 1990). Before you think that is expensive - remember these are the days when a 67Mb (megabyte) harddisc would set you back almost that much - actually an Acorn User advert from 1989 quotes around 700 pounds for a 67Mb "Winchester" harddisc.

The key is expandability. The FileStore comes supplied with two floppy disc drives, each offering around 600Kb storage. My school ran a network in the IT room using a FileStore (don't know if it was E01 or E01S) and thirteen stations (BBCs and Master Compacts, assorted) and all they ever had were the two floppy drives in the server.
If needs grew, if RISC OS machines were added to the network, if the network expanded to cover other classrooms, then the FileStore could have a harddisc added. The older E01 model required a harddisc unit with built-in SCSI controller. The later E01S model had the SCSI controller on the main board. The difference? The E01S could have up to four harddiscs daisy-chained.

Likewise the MDFS. The most basic setup is an MDFS with floppy disc drives. This can be expanded to a maximum of four floppy drives (double or quad density) plus up to four harddiscs (or is it four partitions? I forget...) along with a tape streamer for backup purposes.

Comparitively priced, it was a lot less expensive than other networking solutions. It also had the plus of being a reliable tried-and-tested technology.
Therefore, schools up and down the country that used Acorn computers used Econet. Econet, being robust, saw itself in businesses and factories, the "Cambridge Ring" and so and so.

Copyright © 2008 Richard Murray