Econet - Physical Layer

Physical Layer

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 5-volt 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.

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.