80-Bus News

High Speed Cassette Interface

We have received the following letter and article from W. Van Malderen in Antwerp, which comes as a reply to our request for high speed interfaces. Far be it from us to ridicule our overseas friend’s somewhat idiomatic use of English (and to that end parts of Mr. Van Malderen’s article have been rewritten) but we reproduce his original letter here uncorrected, as in some strange way we feel it encapsulates the spirit of experimentally inclined Nascom 1 owners. DRH

B2000 Antwerp
Belgium

Dear Editor,

Until now I was rather a silent overseas member of the INMC, each time waiting until the next INMC News was published. Then when it arrived, it was again joy for lots of days. Sometimes the magazine came up with solutions for problems I had, sometimes it containt programs that could enjoy me, but for some also my children.

The only thing that stayed very annoying was the slowness of the cassette interface. First I speeded it up to 488 baud, thus doubling the standard baudrate (Nascom 1). Finally I found a promising circuit in a Dutch magazine. Then I thought I couldn’t keep this circuit unknown for the fellow INMC members, and I started to write the additional article. I hope it will help on its turn someone else who struggles with these problems.

To conclude, I wish the INMC News to stay as excellent as it is.

Your sincerely,
W Van Malderen

PS. excuse for possible spelling faults.

Although the Nascom 1 is a very fine machine (and I have no experience with the Nascom 2), I find that the Nascom 1 needs something better than it’s standard cassette interface to avoid frayed nerves during a Read, Write or Verify cycle. An article in the Dutch magazine ‘Elektuur’ [published in the UK as Elector. Ed.] of June 1980 brought the solution to this problem. I shall try to summarize the contents of that article.

Fast cassette interfaces

The most commonly used cassette interfaces employ some form of f.s.k. and Kansas City (CUTS) standards. The following cassette interface uses the rather more professional ‘Manchester II” standard, which has the following advantages:

– Clock reclamation is fairly easy as it is a component of the recorded data
– The circuits are not particularly complex
– The redundancy (efficiency) is 50%

The redundancy figure being 50% means that the baud rate may be equal to the highest tone frequency used, the lowest frequency will be half the maximum baud rate and therefore half the maximum tone frequency used. This means that to use a baud rate to 4800, the cassette recorder has to be capable of reproducing 4.8KHz. Given a recorder with a response of 10KHz, it would be possible to use a baud rate of 9600, assuming the read/​write software can go that fast. The version discussed here and used by myself and several other Belgian Nascom owners, and owners of other Z80 based machines runs at 3900 baud. This speed can be handled by the existing Nas-bug T4 and Nas-Sys software. The system has been found to be extremely reliable even on very large tape files.

Generation of the Manchester II system signal is simple and is done by combining the clock and data signals into a composite signal using a NOR gate, pulses