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. The Acorn A4 laptop used another implementation, in the form of a 5 pin mini-DIN. 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. Network and transport layers Econet used a connectionless transmission model, similar to UDP, with no checksumming or error correction at this layer. Each packet had a four byte header consisting of: The destination station number The destination network number The source station number The source network number A single data transmission consisted of four frames, each with a header as above: The sending station sends a scout packet with a port number and a flag byte The addressed receiving station returns a scout acknowledge to the sender The sending station sends the data The receiving station finishes with a final acknowledge, identical to the scout acknowledge Each device on a network segment is identified by a "station ID" number which serves a similar purpose to a MAC address. The address is not set at time of manufacture but is set manually and a network administrator must ensure addresses do not collide when new machines are installed. In the BBC Micro the station ID is set using jumper pins on the motherboard. Conventionally a file server would be assigned station ID 254 but there is no specific requirement for this to be the case. 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