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Setting the Clock Speed

The speed of the network clock must be suitable for the length of network and the machines you are running. An incorrect clock speed is the most likely cause of an apparently inexplicable "Net error" or "No clock" message.

The clock speed determines the speed of data transfer across the network - if you set a really slow clock speed, you can sometimes almost see the network "thinking" if you try to catalogue the current directory, for instance.

In the past, various tables have been published showing the optimum clock speed for different lengths of network. I have never set much store by this. The best way to determine the clock speed for a given network is by a process of trial and error.

General Principles

The clock speed of any Econet clock is made up of two phases, the mark and the space. When added together, the mark and the space are known as the period.

The way these settings are determined depends on whether you have a Symmetrical or Asymmetrical clock. The Acorn Level 1 Clock and the American style Acorn clock are symmetrical. All other clocks are asymmetrical.

Symmetrical Clocks

The mark phase and space phase are the same length in symmetrical clocks. They cannot be set independently of each other. Instead, the clock rate in KHz is chosen from a set of links inside the clock. Click here for more information on the US-style Acorn Econet clock and speed link setting.

Asymmetrical Clocks

More common is the asymmetrical type of clock where the mark and the space can be set to different lengths. The Acorn Level 3 clock and FileStore clock and SJ Research Clock and SJ Research Bridge Clock are types of assymmetrical clock.

Acorn and SJ Research adopted a different method of setting the speed. Both the mark and space phases are measured in microseconds.

Acorn Asymmetrical Clocks

Acorn clocks allow you to set the mark and space. Added together, these give the period, or overall clock speed. The mark may be 1 or 2 microseconds. The space is a multiple of the mark and may be a multiple of 1 to 15. (A maximum multiplier of 7 is available on the FileStore clock.)

The Acorn Level 3 clock provides space multiplier links of 1, 2, 4 and 8. By setting different combinations of links, the multiplier can be set between one and 15.

Therefore, to set a clock speed of 8 microseconds, you might set the mark to 2 microseconds and the space multiplier to 3 by setting space links 1 and 2. The clock speed is then:

2 + ((1+2) x 2) = 8 microseconds

To work out the network speed frequency of a given clock speed, the speed in microseconds is inverted and then multiplied by 1,000. Our speed of 8 microseconds is thus:

(1/8)*1000 = 125 KHz

It is important to remember that the data on the network is transmitted during the space phase. It is therefore sensible to set a longer space period than mark period. An alternative way of getting a 125KHz clock rate with much longer space period would be as follows:

Set the mark to 1 microsecond. Set the space links for 1, 2 and 4 = a multiplier of 7. This then gives a clock speed of 8 microseconds as before:

1 + ((1+2+4) x 1) = 8 microseconds

Although the clock timing is the same in both examples, in the first example, the clock spends a quarter of the time in the mark phase and three-quarters of the time in the space phase. In the second example, the clock spends an eighth of the time in the mark phase and seven-eighths of the time in the space phase, allowing more time for data to be transmitted.

SJ Research Clocks

To complicate matters, SJ Research brought out a different system for setting the clock speed. Firstly, the timings of the two phases could be refined much more accurately and secondly, instead of setting the mark and then the space as a multiple of the mark, with SJ Research Clocks, you set the mark and total period. The space phase is then the total period minus the mark.

There are two sets of DIL switches on the SJ Research Clock. The top set are for setting the mark, the lower set change the overall period. The mark can be set anywhere between one-eighth of a microsecond and seven-and-seven-eighths microseconds.

The period can be set to anything between one-quarter microsecond and fifteen and three-quarters microseconds.

When setting the SJ Research Clock, always remember that the period MUST be longer than the mark otherwise the space phase will be zero or negative. If this occurs, all you will get is a No Clock or Net Error message.

To set our clock frequency of 125KHz on the SJ Research Clock, we need to set the period to 8 microseconds. This is done directly on the SJ Research Clock by setting the Period DIL switches. We must then set the mark, which could be anywhere between an eighth and seven-and-seven-eighths microseconds. The space should be set as long as possible. With the SJ Clock, the space is the Period minus the mark, so setting the mark to, for example, 1 microsecond, will give us a space of 8-1 = 7 microseconds.

Trial and Error

The above is a guide to the general principles behind clock speed setting on an Econet network. It is a simplified guide and it is impossible to determine the best clock speed for any given Econet without a certain amount of trial and error. No doubt a team of scientists at the University of Woolamaloo could spend many years and many billions of taxpayers' money trying to devise a definitive answer.

If you're baffled by the above then don't panic. You don't really need to understand how the clock settings work to run you Econet. If in doubt, I suggest using the Acorn recommended network speed of 200KHz. To do this, work out which clock you have and do the following:

Acorn Level 1 Clock (symmetrical): set the speed link to 230K which is the nearest to 200KHz available on this clock.

Acorn Level 3 Clock (asymmetrical): set the mark to 1 microsecond and the space multiplier to 4.

SJ Research Clock (asymmetrical):set the mark to 1/2 microsecond or 1 microsecond and the period to 4+1=5 microseconds.