Acorn Level 3
Econet Clock
This is the circuit board inside the Level 3 Clock, or "Econet Clock 2", as it announces itself.
Out is the Motorola 14411 driver chip, replaced with some 74-series logic chips to generate the clock signals.
There are two Econet sockets and two sets of links for setting the clock speed.
The Level 3 clock is the first asymmetrical clock; that is, it can generate a clock signal with different lengths to the two phases, the Mark and the Space. The earlier Acorn clock was symmetrical, having an identical length to the mark and space phases.
This was highly limiting as the network transmits data during the space phase, so half of the potential data transmission time was lost with the Level 1 clock. With the Level 3 clock, it is possible to set the space to a longer period than the mark, to maximise the time available for data transmission and therefore increase the overall speed of the network.
This is achieved by setting the mark via link 1 at the top right, to a timing of either 1 or 2 microseconds.
The space is then set usings links 2 to 5, on the left hand side next to the crystal. The space is not set in terms of a length of time, but as a multiplier of the mark setting. Links are available for multipliers of 1, 2, 4 and 8, and any combination of these can be used to set the value of the multiplier.
The space can, therefore, be anything from a multiple of 1 to a multiple of 15 of the mark. A space multiplier of 1 will produce a symmetrical clock which eliminates the advantage of extending the space to longer than the mark.
In practice, a mark of 1 microsecond and a space multiplier of four or five will be the optimum setting.