Acorn (type 2) clock

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The Acorn (type 2) clock box

The Acorn clock.The type 2 clock (also known as the Level 3 clock - confused yet?) is contained within a black box with two sockets. It is an asymmetrical clock - the MARK and SPACE durations can differ.

The asymmetrical clock allows for better network performance (compared to the type 1 clock) as it allows the 68B54's to operate more reliably. Refer to the section on determining clock speeds for why this is so.


Setting the clock

Within the type 2 clock, there are two sets of links that are important.

The first of these is on the top-right of the clock (above the Acorn logo) and it is marked LK1. This allows you to set the MARK duration to 1µs (if the link is in the upper position) or to 2µs (if the link is in the lower position). Those are your only options.

The second set of links is on the left hand side, just above the centreline (near the shiny rectangular metal can). These links (LK2 to LK5) allow you to set the SPACE duration:

   LK5  o o  1µs
   LK4  o o  2µs
   LK3  o o  4µs
   LK2  o o  8µs
The default setting is for a 1µs MARK and a 4µs SPACE.



This clock is similar in design to the FileStore clock. It takes the output from an 8MHz crystal is divided to make 0.5µs pulses, which are fed into latches and counters configured by your links.
The primary difference is the output power of the clock, so it will work with reasonable-sized networks (unlike the maybe-six-metres that the FileStore's clock can manage).

Copyright © 2007 Richard Murray