Econet was a network used by the BBC Micro, conceptually similar to Ethernet, two orders of magnitude slower and correspondingly cheaper.

According to https://en.wikipedia.org/wiki/Econet it transmitted one bit per clock cycle.

Does this mean every computer or other device on the same network needs to have the same clock speed? Or at least in a simple ratio? That is, given a network of BBC Micros (2 MHz), I would expect to be able to design a card to connect a Commodore 64 (1 MHz) to that network, but an Atari 800 (1.79 MHz) would seem hopeless (short of giving the Econet card its own clock and running asynchronously with the computer, but that would seem outside the scope of a network card for an 8-bit computer). Is my intuition on this correct?

  • 1
    "Clock" and "clock" isn't necessarily the same thing ;) – tofro Aug 25 '18 at 13:21
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    Not System Clock, it's Serial Clock - there is also a clock line on the interface, everything is synchronous to this clock. After all, a computer is wa holw bin full of different clocks, the CPU clock only being one of them :)) – Raffzahn Aug 25 '18 at 13:42

In Econet, using synchronous serial transmission, the sender provides the clock together with the data. The receivers need to synchronize with that clock. And the network clock doesn't (necessarily, although it might be somehow derived from the same clock) need to be connected in any way with the system clock.

In the BBC Micro, this clock (and the whole Econet interface) is being provided by the Motorola 68B54 Data Link controller chip, which relies on an external baud rate generator that provides the clock.

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Econet networking was first developed for Acorn's System line, which had a 6502 CPU clocked at 1MHz. It was later used in their ARM-based Archimedes machines with 8-25MHz CPUs, as well as the RISC PC line which pushed into the hundreds of MHz. So the system clock clearly doesn't need to be identical for all stations on the network.

There is, however, another clock needed on an Econet network: the network clock. This is a synchronisation signal transmitted to all Econet stations on the network, independently of data transmissions (which are carried on a separate pair of wires). System, Atom, and early BBC Micro Econet interfaces included a built-in clock generation circuit, but as only one clock signal is needed per network, it was removed from BBC Micros from Issue 4 onwards. The preferred approach was to generatle the clock signal with a separate device at the middle of the network, which was referred to as an Econet clock.

The frequency of the clock signal dictates the maximum transmission rate on the network. Naturally you'd want to use the fastest clock rate possible, but the length of your network and the processing speed of the connected stations will limit this.

In their 1984 Econet Installation Guide, instructions are given for varying the clock speed between 31kHz and 500kHz, mainly by changing the length of the clock's low phase (space) relative to the high phase (mark). Acorn recommend a maximum clock speed of 200kHz in this guide. They note that for a 500m network that value should be reduced to 180kHz, but that 300m the speed of the station hardware is the limiting factor: the Acorn Electron is specifically mentioned as requiring a slower clock speed.

In later years, with the introduction of the faster Archimedes machines, faster clock speeds (and transmission rates) could be achieved, but only if slower devices were removed from the network. One solution was to run a separate, slower network for old Beebs, and connect it to the faster network via an Econet Bridge.

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