Determining clock speeds

  There are three stand-alone Econet clocks:

• Acorn Level 1 clock, in a grey/grey case with a single socket.
  The use of the Level 1 clock is not recommended due to its symmetrical output.
• Acorn Level 3 clock, in a black case with two sockets (as shown above).
• SJ Research clock. This is the most configurable of all the clocks.

There are three parts of the clock signal that you need to be aware of:

• MARK - this is when the clock signal is high (on).
• SPACE - this is when the clock signal is low (off).
• PERIOD - this is the speed of the clock, which is MARK + SPACE.

Data is transmitted on the falling edge of the mark. Therefore, within a certain period, you ideally want the mark as short as possible and the space as long as possible. The faster your clock goes, the faster you can transfer data across the network. However this is modified by three factors:

• Longer networks require a slower clock speed to prevent spurious signals and data collision in the cable.
• Poorer quality cable requires a slower clock speed. The best cable is dual twisted pair with shielding. Some people, on the other hand, use 6-core telephone cable!
• The minimum time required for your stations to recognise the clock.

  The duration required for each type of machine to recognise the clock is shown below, along with experimental times taken from my network.

   BBC micro or Master . . . .  4µS  ; minimum 4µS         (250kHz)
   FileStore . . . . . . . . .  4µS  ; only likes its own clock!?
   SJ research MDFS  . . . . .  3µS  ; works down to 2µS   (500kHz)
   SJ Econet bridge  . . . . .  3µS  ; works down to 2µS   (500kHz)
   RiscOS computers  . . . . .  2µS  ; works down to 1.5µS (~660kHz)

   Maximum period duration . . 13µS  ; ( 70kHz)
   Acorn default clock setting  5µS  ; (200kHz)

       2 × d
   S = -----
       v × c

   Where:

     d = distance of the longest side of the network (in metres)
     v = velocity factor - assume 0.5 for most cables
     c = the speed of light - 300,000,000 (3×108)

   For example, a 600m network with the clock 200m from one
   end and 400m from the other.

           2 × 400
   S = ---------------  =  5.33333333E-6, rounded up to be 6µS
       0.5 × 300000000

   Therefore, your SPACE is 6µS, try your MARK to 1µS,
   which gives you a PERIOD of 7µS - a frequency of 143kHz.

The best thing to do is to set the above recommended clock speed, and then increase it in small steps until you the network becomes unstable; then slow it down a step.
You should, on average, be able to shave at least a µS off of the recommended speeds, except for 6502 based equipment such as BBC micros and FileStores.
The MDFS cannot quite keep up with RiscOS machines, but it isn't far behind - and you will find your network is quite comfortable operating a tiny bit slower in order to allow the MDFS to be used.

  Please be aware, however, that the MDFS may require a slower speed than it seems. My MDFS is quite happy serving using a 0.5µS mark in a 2µS period. When I come to use the FAST terminal to talk directly to the MDFS firmware, it fails on anything faster than a 1µS mark in a 4µS period... Remember to check for things such as this.

  If you have hooked in the MDFS on a seperate network using the SJ bridge, it is a trivial matter to set the bridge to Admin mode and alter the speed as applicable - it is almost fun!
Those of you using a clockbox, who are handy with a soldering iron, might like to rig up some kind of flip switch on the clockbox so you can instantly select a fast or slow clock.
I did this to my FileStore in the days before I had a clockbox. Switch to the left, lethargic mode for the FileStore. Switch to the right, a little quicker for the A3000.

If you consider this, I have a few suggestions:

• Don't solder your switch wires directly to the clock links. Solder each to a link jumper, then attach the link jumpers to the relevant links. This allows you to alter the links at a future date.
• You can get away with three wires, three link jumpers and no trying to solder two wires to one link jumper. It makes the switching easier too...
  One side of the clock selection pins will be a 'common'. You attach one link jumper to this. Your other two jumpers go to the other side of the selected speeds.
  For example, on an Acorn issue 2 clockbox:

   LK5    o o  1
   LK4    o o  2
   LK3    o o  4
   LK2    o o  8
  

  The left-hand pins are all connected, and the right-hand pins (when connected to the left) select a clock speed. Thus, to switch between 1µS and 4µS, you might do something like:

        .--------------------------------.
        |                   (fast)       |
    LK5 | o XX---------------------O\    |
    LK4 | o o  2                     \O--'
    LK3 | X XX---------------------O
    LK2 `-X o  8            (slow)
  

• As the SPACE is where the data transfer takes place, you want to be able to narrow down the MARK as much as possible. The Acorn clocks allow you to set this to 1µS or 2µS, which isn't very flexible and limits your overall data rate. With an SJ clock you can set the MARK short to, say, 0.5µS and the PERIOD to 1.5µS which gives a 1µS SPACE. As the Acorn clock only allows you to set the MARK and SPACE to a minimum of 1µS, you are instantly running a third again as slowly.