Acorn System Econet Interface
The Acorn System was based around a rack into which various cards were plugged. This Econet card was one of them and is the earliest form of Econet interface. Introduced around 1980, it looks surprising similar to the "podules" which would become a feature of the later 32-bit Acorns.
All the integrated circuits on the board are socketed, which is something which would have been handy on later Econet modules.
The 5-pin DIN socket is not soldered onto the board but is wired in separately. Perhaps at this time the Econet connecting lead was not finalised.
Despite being an early Econet card, the board has all the familiar components, including the 68B54 controller (in this case a slightly slower 6854) and 75159 line driver.
This Econet card dates from the time before external clock boxes and terminators were invented. Instead, provision for clock generation and termination was provided on the Econet interface, a system which lasted until Issue 3 of the BBC micro motherboard.
The card above in fact has the clock circuitry added at IC6 and IC7 and some additional resistors and capacitors. The clock speed would have been set by selecting the clock rate in KHz using the links next to IC7, rather like the system used on the Level 1 Clock.
The remaining empty spaces for resistors and capacitors are for network termination.
Many thanks to Jules Richardson for this picture of the issue 1 Acorn System Econet card from 2004, which I updated from the original in April 2018 to increase the size from 800 pixels to 1800.