The trivial way to accomplish this is to use a 64Kx8 SRAM and simply disable it
when a ROM or other memory device is selected.
Scratch building a system with 64Kx8 SRAM and a 64Kx8 EPROM that copies itself
in to the RAM, then disconnects itself, would take, perhaps an hour or two. A
video circuit, with or without an RF modulator, and an FDC might take a mite
longer.
Sifting through the parts in a kit to verify they're all there and studying the
instructions so you get it right the first time would probably take longer than
that. Confidence in the end=product is a valuable part of the mix, however.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Saturday, September 29, 2001 1:09 PM
Subject: Re: ZX81 kits (was Re: Keep or assemble???)
> >
> > > > The ULA does a lot more than handle the display ;>)
> > >
> > > CPU clock generation (which would be useful). Address decoding (which I'd
> >
> > > want to modify if I was changing the amount of RAM).
> >
> > Very interesting -- how much RAM are you talking about adding that you have
> > to modify the address decoding?
>
> Well, assuming I could fit Forth into 8K, then I see no reason not to
> have 56K RAM :-).
>
> Maybe I could use the existing address decoder, but I'd want to extend it
> for the extra RAM so it wouldn't be much more work to design the whole
> thing from scratch...
>
>
> >
> > > > Of course not ;>) But a ZX81 kit is still faster than starting from
> > > > scratch.
> > >
> > > Is it? It's not going to take long to wire-wrap a clock circuit and
> > > address decoder, and stick in the Z80. It depends _very much_ on how much
> >
> > > of the ZX81 you want to use...
> >
> > Which in turn depends on what the intended use of the finished device is.
>
> Of course...
>
> > Since I want a "general purpose" computer the ROM BASIC functions are very
>
> Sure, if you want to kepe the ZX81 BASIC then it makes sense to start
> from a ZX81. As I said, I'd probably not want that...
>
> > convenient, and fast if called directly. Other applications of the Z80
> > would not need this code so a quickie Z80 board would be fine.
> >
> > > > Try desoldering the RF modulator from a ZX81 board ;>)
> > >
> > > Well, I had never tried to do it before, so I grabbed a ZX81,
> >
> > What, do you have a pile of these?
>
> I have a couple I think. I knew where one of them was, and I don't
> normally mind dismantling computers for experimentation. I was
> sufficiently confident of my soldering skills to believe that either I'd
> be able to remove the modulator without damage or that I'd stop
> attempting to remove it before any damage was done if there was a problem.
>
> >
> > > unscrewed
> > > the case and attacked it with soldering iron and sucker.
> > >
> > > What's the catch?
> >
> > In the US versions which have 3 thin wires, they sometimes become brittle
> > and break.
>
> The UK version only has 2 wires. Still, I guess they could break off as
> well. It wouldn't be that hard to repair that sort of damage The
> modulator can can be taken a part and the video input wire re-soldered to
> the PCB inside. I am sure the power wire to the feedthrough capacitor
> could be repaired as well.
>
>
> >
> > Tony (and others), a question: in your opinion, which microprocessor was
> > the most well-designed (even if not implemented)?
>
> For the 8 bitters, probably the 6809.
>
> For 16 bitters, one of the PDP11 ones (T11 or something), possibly.
>
> -tony
>
>
Received on Sat Sep 29 2001 - 16:17:47 BST