28 Sep
2001
28 Sep
'01
4:37 p.m.
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?
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.
Since I want a "general purpose" computer the ROM BASIC functions are very
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, 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.
Tony (and others), a question: in your opinion, which microprocessor was
the most well-designed (even if not implemented)?
Glen
0/0
29 Sep
29 Sep
7:09 a.m.
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9:17 a.m.
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(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Saturday, September 29, 2001 1:09 PM
Subject: Re: ZX81 kits (was Re: Keep or assemble???)
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...
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.
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? 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. 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.
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.
Try
desoldering the RF modulator from a ZX81 board ;>)
Well, I had never tried to do it before, so I grabbed a ZX81, 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.
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
9:48 a.m.
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11:43 a.m.
see below, Plz.
Dick
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Saturday, September 29, 2001 3:48 PM
Subject: Re: ZX81 kits (was Re: Keep or assemble???)
They're more common here, too, but the 64K parts are easier to use once
one's
got one. Once you have the tools, an in-situ-programmable CPLD is probably
adequate for nearly any SBC design, and small ones are just a few bucks, even
The logic on larger SBC's, e.g. the Ferguson Big Board, wouldn't be
difficult to
reduce to another ISP CPLD + a small RAM and a Character Generator EPROM, e.g.
2716, or the like. It wouldn't take much to derive the CPU clock from the video
circuit, since its dot clock would be on the order of 14 MHz or so. If you were
really determinedly minimalist, you would probably use the same oscillator to
drive the disk interface, the video, and the CPU. It might require quite a few
compromises, but ... with today's multi-output PLL's ... If you were totally
determined to use an NTSC- or PAL-compatible monitor, it might be tricky, but if
you use an old PC Mono monitor, and time the video circuitry such that it runs
>
> 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.
Sure... But 32K*8 SRAMs are more common than 64K ones in the UK, so I'd
probably use a couple of those. And then I'd want to make sure that the
RAM/ROM was only enabled when MREQ was asserted. Not hard, of course.
It's going to be a couple of chips at most, so it's not worth keeping the
ULA just for that...
from places like DigiKey.
>
> 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.
As I've said a couple of times before, I can make a video circuit with
somewhat better capabilities than the ZX81's built-in video (40*25 text,
block grapghics, colour) in 2 or 3 chips. Add another chip or so to
encode the outputs to PAL (although I'd probably prefer to feed the RGB
signals into a TV's SCART socket these days...)
from the 16 MHz that the PC mono circuit uses ... The
FDC can run from that as
well, and the Z80A likes 4 MHz, The Z80H likes 8 ...
-tony
30 Sep
30 Sep
1:39 p.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
Tony Duell skrev:
As I've said a couple of times before, I can make a
video circuit with
somewhat better capabilities than the ZX81's built-in video (40*25 text,
block grapghics, colour) in 2 or 3 chips. Add another chip or so to
encode the outputs to PAL (although I'd probably prefer to feed the RGB
signals into a TV's SCART socket these days...)
What chips would you use? I assume that the Speccy uses one chip for its
entire video generation, but where would you split the functions?
----
(update a few minutes later, aren't off-line readers a wonderful thing?)
I went and opened up my own seemingly-dead Spectrum... Wait, that's no ZX81.
I've been thinking about SZ Spectrums all the time.
Oh well, I might just ask anyway, since I've opened it up...
What is that chip next to the Z80? It's labelled PCF1306P.
And why is there a switch glued on top of the Z80 with wires going to the
character ROM? Since it's a localised character ROM piggybacked on top of the
one soldered to the motherboard, could it be for switching charsets?
--
En ligne avec Thor 2.6a.
Allt ?r under kontroll, och caps lock ?r bredvid.
1 Oct
1 Oct
8:31 a.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
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10:51 a.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
Tony Duell skrev:
> Tony Duell skrev:
>
> >As I've said a couple of times before, I can make a video circuit with
> >somewhat better capabilities than the ZX81's built-in video (40*25 text,
> >block grapghics, colour) in 2 or 3 chips. Add another chip or so to
> >encode the outputs to PAL (although I'd probably prefer to feed the RGB
> >signals into a TV's SCART socket these days...)
>
> What chips would you use? I assume that the Speccy uses one chip for its
> entire video generation, but where would you split the functions?
I should have said _standard_ chips. Once you start
allowing ULAs (as the
ZX81, Spectrum, Oric, etc all used) then it's easy to do video in a
couple of chips.
Of course I don't expect you to be making your own ULAs. =)
I just wondered if you'd take some off-the-shelf VDU controller or something a
bit more open-ended.
Didn't we go through this a couple of weeks back?
The 2 chips I would
start with are an SAA5243 Teletext display chip (or an SAA5245 for the
States) and an 8K*8 static RAM (6264). You need to get a 6MHz clock from
somewhere (maybe another chip for that, if you can't also use it for the
CPU clock in some way) and maybe you want to buffer the video output lines.
Teletext? That's a funny idea.
> (update a few minutes later, aren't off-line
readers a wonderful thing?)
> I went and opened up my own seemingly-dead Spectrum... Wait, that's no ZX=
> 81.
> I've been thinking about SZ Spectrums all the time.
> Oh well, I might just ask anyway, since I've opened it up...
> What is that chip next to the Z80? It's labelled PCF1306P.
If it's another 40 pin chip, then that's the
custom ULA chip that
includes most of the glue logic.
Is that like a lot of 74138s or something?
> And why is there a switch glued on top of the Z80
with wires going to the
> character ROM? Since it's a localised character ROM piggybacked on top of=
> the
> one soldered to the motherboard, could it be for switching charsets?
Does the Spectrum really have a separate character
generator ROM. Doesn't
sound right to me...
I wouldn't know, but this is the chip which somehow enables Swedish
characters. Is it a patched BASIC ROM?
--
En ligne avec Thor 2.6a.
The most useful program will be continually improved until it is useless.
11:46 a.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
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6 Oct
6 Oct
1:17 p.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
Tony Duell skrev:
> >> What chips would you use? I assume that
the Speccy uses one chip for its
> >> entire video generation, but where would you split the functions?
>
> >I should have said _standard_ chips. Once you start allowing ULAs (as the
> >ZX81, Spectrum, Oric, etc all used) then it's easy to do video in a
> >couple of chips.
>
> Of course I don't expect you to be making your own ULAs. =)
No, and you know that _I_ wouldn't suggest using an
FPGA. Not for a 1-off
project, certainly....
How in the world does one make those, anyway? I haven't got the slightest how
one could "burn" devices which are capable of even replacing a CPU. Does one
"burn" at all?
> >Didn't we go through this a couple of
weeks back? The 2 chips I would
> >start with are an SAA5243 Teletext display chip (or an SAA5245 for the
> >States) and an 8K*8 static RAM (6264). You need to get a 6MHz clock from
> >somewhere (maybe another chip for that, if you can't also use it for the
> >CPU clock in some way) and maybe you want to buffer the video output
> >lines.
>
> Teletext? That's a funny idea.
What's the problem? It gives be a simple 40*25 text
display in a handful
of chips...
No problem really, it's just so simple that it seems a bit unorthodox.
When you think about it, Videotex (Prestel, Micronet et al for you in the UK)
looked very much^H^H^H^H^H^H^H^H^H^H^H^H^H^Hwas identical to teletext. And the
6845-generated eight-colour ABC 800 screen looks very similar to that as well.
> >If it's another 40 pin chip, then
that's the custom ULA chip that
> >includes most of the glue logic.
>
> Is that like a lot of 74138s or something?
It's rather more than that (I assume that's the
address decoder part,
which is in the ULA). It also handles the DRAM timing, most of the video
display, and so on.
I thought one of the selling points of the Z80 was its built-in ability to
handle DRAM. But perhaps it's lacking the 6502's ability to easily share
memory with other devices by only doing memory accessing every second or
fourth cycle.
But what does glue logic really mean?
> >Does the Spectrum really have a separate
character generator ROM. Doesn't
> >sound right to me...
>
> I wouldn't know, but this is the chip which somehow enables Swedish
> characters. Is it a patched BASIC ROM?
I suspect it is. Certainly if the keyboard layout is
different then the
BASIC ROM must have been modified.
Couldn't Sinclair have socketed the BASIC? It must have been a lot of work to
piggy-back it onto each imported Spectrum, unless the customers had to do it
by themselves.
--
En ligne avec Thor 2.6a.
G? med i SUGA, Swedish Usergroup of Amiga!
WWW: http://swedish.usergroup.amiga.tm/
BBS: 08-6582572, telnet://sua.ath.cx:42512
7 Oct
7 Oct
7:31 a.m.
New subject: ZX Spectrum internals (Was: ZX81 kits)
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