In essence an Econet network is just a cable linking a collection of BBC computers together. Clearly this means that networked computers can communicate with each other and also with resource servers. A resource server is a computer attached to the network which enables other networked computers to share devices such as disc drives and printers. Obviously the advantage of this is that it is not necessary to equip every computer with its own disc drive and printer. Moreover, software is available to all computers simultaneously.

Thus it can be seen that an Econet generally serves three functions:

• It provides shared disc storage on one or more file servers
• It provides a physical communication link thus enabling such network application programs as electronic mail to be employed
• It allows all stations on the network access to facilities attached to a single machine e.g. a modem.

Below is a typical network layout:

It is apparent from the above diagram that Econet was constructed using the bus topology. Furthermore, it should be explained that each station attached must possess an ECONET INTERFACE and a NETWORK FILING SYSTEM (NFS) chip.

Econet Components

Basically an Econet system consists of a single cable with sockets placed at intervals along it. The actual physical conduit consists of five wires:

• A twisted pair for data signals
• Another twisted pair for clock signals
• A final wire to provide earth

Each socket is connected to the cable in parallel hence messages are received at virtually the same time by all stations.

Clearly, it is advisable to have some sockets embedded into the cable than there are physical terminals - for the reasons of expansion and also so that a station could be removed from one location and attached elsewhere, without affecting the overall integrity of the network.

Econet is a synchronous system and therefore it is clock controlled. A CLOCK GENERATOR produces a clock signal, required for timing purposes, which consists of a stream of square waves of constant frequency. Occasionally a 'No Clock' error message can be generated and this is generally the result of a clock signal becoming corrupted or it may even be as a consequence of an alteration in the frequency of the clock signal.

To each end of the cable a terminator is attached, principally to eliminate signal 'bounce' (when signals are reflected along the cable once again and hence causing interference with other signals).

Incorporated into the network will be at least one FILE SERVER, which provides a filing system for all the stations on the network, allowing them to load and save files from disc. File servers are available form a variety of sources and can be classified as:

• Level 1 - simply a program which allows the floppy disks on a machine to be accessed by anyone on the network.
• Level 2/3 - a more sophisticated file server which provides the following functions:
  • random access file
  • a hierarchical directory structure
  • a password system
  • almost unlimited number of files on a disk
  • date stamping available on all files
  • backup
  • file control attributes
  • a library directory

On the same or on a different machine entirely can be the PRINT SERVER. Common print servers provide SPOOLING facilities which enable them to create a queue of those waiting to print.

The Econet Interface

The Econet interface chip is either placed directly into the main printed circuit board or in some cases intermediately into an Econet Card. The fundamental component of the interface is known as the Advanced Data Link Controller (ADLC).

The ADLC is a complex chip, which performs the main interactions between the computer and the network, and carries out the following functions:

• It interrupts the processor when a message is detected on the cable and the processor must then decide whether it is addressed to it.
• It converts the serial data received from the network into parallel form for the processor and vice versa
• It checks messages are valid and correct - a valid message must have a specified structure
• It adds header and trailer information during transmission so the message constitutes a packet.

The Econet Packet

As in all networks, information on an Econet is sent in the form of a packet, built and dismantled by the ADLC.

The main elements of the packet are:

• Opening Flag - always has the value 01111110
• Receiver Address - a two byte field allowing 65536 stations to be addressed - at present only one byte is employed and the other is reserved for accessing gateways.
• Sender Address - similar to the above
• Port Number - a single byte which informs the receiving station what type of message is in the packet
• Data Block - can be any length
• Checksum- a two byte number
• Closing Flag - same format as opening flag (when this is encountered the receiver knows that the packet is now complete)

Collision Detection

In broadcast networks the main difficulty in data transmission occurs if two stations attempt to send information simultaneously. Clearly, one method to avoid this is for a station to check that the network is clear before transmission. However, in the time interval between checking the network is clear of traffic and actually transmitting the data, there is a possibility that another station could attempt to transmit. If this developed then data on the network would be corrupted. As the Econet Interface includes a collision detector circuit the ADLC is aware that a collision has occurred. The stations would then terminate their transmissions and wait for different periods of time before attempting again, the time depends on their station number.