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Thread: bigdjiver - Kempston Joystick King!

  1. #1
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    bigdjiver - Kempston Joystick King!

    Quote Originally Posted by bigdjiver
    The ads for my shop offered a computer, joysticks, some games and some other accessories - a "bundle". At the time just about everybody else sold those products as single items. I was well ahead of my time there. The computer I sold did not have a joystick option, so I wired joysticks across the keyboard matrix, giving me a unique selling point. I believe that Sinclair should have used that technique with the Spectrum which would have allowed it to use two joysticks for about 50p on the cost. Instead he opted for an expensive add on interface which she could not deliver on time. I spotted that market opportunity, and told Abtaar Pandaal of Kempston Microelectronics how to fit joysticks to the Spectrum. A few oldies might remember the Kempston Joystick interface.

    Here we had an un-authorised add-on, and a chicken and egg situation. Nobody would buy those joysticks because there were not any games set up to use them. None of the games writers would write programs to use them because nobody had the joysticks. In the space of two hours on a wet mid-week afternoon I designed the interface, wrote a demo program, and solved that marketing problem at no cost. Mr. Pandall financed racing cars on the income from that.
    As an owner of a kempston joystick and interface, I'd love to know how you just "Design an interface" in an afternoon?

    It was a great product. You must have made some serious cash yourself bigdjiver, no?

    Will

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    Lovely Moderator ducasi's Avatar
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    Re: bigdjiver - Kempston Joystick King!

    We've had this discussion before on the forum... This is all from memory going back more than 20 years, so I might have a few details wrong... Apologies if I do.

    I'm sure Big D's design was good, but the implementation of the design was flawed.

    The idea is that port 31 was the joystick, so an "IN 31,a" would read the joystick.

    The techy amongst us will know that to decode this address is easy – in a sixteen bit address space the bottom five bits are all on, and the top 11 all off. The top 8 bits (the high byte of the 16 bit address) of those 11 were usually ignored by the hardware, so that just left the top 3 of those 11 to worry about.

    However, the Kempston Interface didn't bother checking the top 3 bits – so then any address with the bottom 5 bits on would read the joystick. This wasn't so bad... Things worked – there were no other devices that wanted all the bottom 5 bits on...

    To check the bottom 5 bits all you needed to do then was "AND" them together.

    Now we come to the second "however"... Because, for some reason rather than using a couple of chips with AND gates on them, it used chips with OR gates – and ORed the bottom 5 address lines together.

    Therefore any "IN" of any address with one of the bottom 5 bits set would read the joystick.

    Thus, if you were trying to read from some other device that used an address that the Kempston interface thought was its own, it could cause problems.

    Fortunately nothing too serious, but I had some issues with it when I was developing some of my own programs. Unfortunately I can't remember the details.

    Still – the Kempston Interface made a big difference within the ZX Spectrum world, and so the designer does deserve respect.
    Last edited by ducasi; 7th-November-2006 at 01:50 PM.
    Let your mind go and your body will follow. – Steve Martin, LA Story

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    Re: bigdjiver - Kempston Joystick King!

    Abtaar Pandaals company was selling an 8255 interface for the Spectrum which allowed experimentors to play with switches and LEDs etc. The addressing scheme and design of that was his. All of the interfacing that I did was to connect one of the ex-video games Joysticks that I was buying from Voltmace to his existing 8255 interface. I still have the original joystick. Voltmace later produced the PCB's for him. Whilst my engineer was wiring the joystick interface I was writing the bit of Speccy code to read the 8255 input from the joystick, and tell him how to get around the catch 22 marketing problem.

    I made very little from this, I was waiting for the Bailiffs at the time, Dragondata had broken their contract and I was being forced into bankruptcy. I was quite deliberately passing on this information for "the good of the nation". I saw it as vital for the British computer software industry that the Sinclair Spectrum was a success as a games machine. Abtaar did not believe that the product would sell in any real volume. His first production run was 24. I knew it could make a lot of money, but did not know at the time how severe Sinclairs problems were.

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