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From: Richard Atkinson
Date: 2000-08-07
Subject: Re: Plus 4 Database Update

On Mon, 7 Aug 2000, Tim Böscke wrote:

> > Not quite, but a similar level of complexity. Do you have details of this
>
> uhm well.. swap one ls257 for a ls153 :)

I'll explain my expansion's principle; an 8 bit latch, two four-to-one multiplexors and address decoding logic to map the latch to $FD7x. The latch could be a 6529 for read / write capability, but as these are rare I specified a 74 series part which would be write only. The 8 bits are divided into four groups of 2 bits; for each of the combinations 00, 01,
10 and 11 of A15 and A14, the hardware generates A17 and A16 which allows four different banks selectable in each 16K block. I like to think it's quite versatile, however, the main reason for the design is its simplicity, so if there is something simpler it will have effectively been obsoleted.

> > very simple Hannes expansion? I don't think I've ever seen a diagram of a
> > circuit that simple. The ones I have seen have been far more complex.
>
> The basic extension maps bank 3 to $0000-$1000 or to $0000-$4000 (choosable
> with one bit) and bank 0-3 into the space above this. Thats it.
> There is another slight extension to redirect all TED DMAs to bank 3 when
> a bit is set.

Please, can you send me or point me to fuller details of this expansion,
such as circuit diagrams?

> > The reason I developed this alternative expansion is that I was already
> > developing one for the C64 and C128 and it uses the same principle, so I
> > can re-use the hardware between the machines. If it really is as simple as
> > the simplest Hannes expansion then I will of course abandon development
> > and build a Hannes 256K plus/4 instead.
>
> Probably the better choice. Getting software support for a new plus/4
> 256kb extension is nowadays a bit difficult :)

I would patch older programs myself, of course, and even then I'd only get time to do the ones I'm interested in. I understand there's a large amount of 256K-supporting plus/4 software, so perhaps I've underestimated the enormity of this task. However, I've never encountered much on my searches.

Richard

--
Richard Atkinson Software Engineer Tenison Technology EDA Ltd http://www.tenisontech.com/