On Oct 7, 2:17, Iggy Drougge wrote:
> >> >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.
The Z80's special ability in handling DRAM is merely the presence of an
on-board counter which increments once in every instruction cycle, and
which is output onto the address bus once per instruction. This is only
part of the requirement, however (see below). The refresh counter means
the chip is guaranteed to cycle through the addresses (with no omissions)
in a given period. This is exactly what you need for DRAM refresh. Of
course, if you have a video circuit accessing the same DRAM, it too will
cycle through the address range (or part of it) and achieve the same end,
so the Z80 refresh is only a useful bonus if you don't have video accessing
your DRAM. Anyway, it's only a 7-bit counter, which is fine for 16Kbit
DRAM but not for most 64Kb devices.
The downside is that a Z80 uses the clock in a different way from a 6502.
A 6502 does some things on one half of the clock cycle (like accessing
memory to read instructions or read/write data) and other things (internal
operations) on the other half cycle. During the half-cycle when it doesn't
need memory access, it releases the bus, leaving it free for use by other
devices (eg video). A Z80 uses successive cycles (of a faster clock)
instead of alternate half-cycles for different purposes and there is no
clock state when it's guaranteed not to be using the bus. However, there
is one (whole) clock cycle when it doesn't need the bus, and that's the
refresh period; it's just more complicated to decode that than to just use
the phase of the clock (high or low) to enable your video access (or
whatever). It's also less suitable than the 6502 since Z80 instruction
cycles vary in length and therefore the interval between refresh cycles is
not constant. The interval between successive half cycles of a 6502 clock
is (usually!) constant.
I said the refresh was only part of the requirement. That's because DRAM
normally splits the address into two parts, row addresses and column
addresses, to address a given cell in the memory matrix. Usually these two
parts of the address are multiplexed onto the same pins of the IC.
Therefore, on the first part of the memory access cycle, you put the row
addresses on the pins and signal this to the DRAM with the /RAS (row
address select) signal. The IC latches these address bits internally, and
after some time period, you turn off /RAS, replace the row address bits
with the column address, and apply the /CAS signal. Neither the Z80 nor
the 6502 have any special feature to simplify this (unless you count the
fact that you have two clock phases per memory address on a Z80). So in a
Spectrum, ZX81, etc, the ULA takes care of the address multiplexing and
RAS/CAS timing.
> But what does glue logic really mean?
Just slang for the assorted logic that does address decoding, signal
buffering, RAS/CAS generation, etc. In a typical system with, let's say,
ROM, RAM, processor, some I/O ports, perhaps a DMA device, each section
that connects to the address/data bus (and perhaps the control bus too, if
DMA is involved) would have its own set of buffer ICs, each being enabled
only when the relevant section is accessing the bus. The glue logic
includes all the decoding/multiplexing of the address and control signals.
If you think of a system as CPU, RAM, ROM, I/O ports, then the glue logic
is all the extra circuitry you need to "glue" them together into a working
system.
> 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.
Sinclair never used sockets if they didn't have to, for reasons of both
cost and reliability. If there was a special Sinclair-original ROM for
Scandinavian countries, I would have expected that to be factory-fitted
instead of the standard ROM. It's possible they retrofitted something like
that -- it might be cheaper to piggyback a new ROM instead of removing the
old one -- but I would guess that what you have is something done by the
original owner, in order to have both systems. Sinclair would be most
unlikely to fit something as extravegant as a switch.
--
Pete Peter Turnbull
Network Manager
University of York
Hey folks. I've got four or five SGI Personal Iris machines
here...4D20, 4D25, and 4D35, in various configurations and in various
states of operability. I also have three Macintosh Quadra 950s. I
offer them for free to any interested party for pickup in Laurel, MD
within the next two weeks or so. I am moving, and they need to go.
In addition, there's a list of [non-free] stuff that I have available
at http://www.neurotica.com/available.html.
-Dave
--
Dave McGuire
Laurel, MD
While visiting my electronics shop, I found a collection of largish cards
(circa 15?25 cm?) made by WANG. There were at least two video cards - one IBM
mono emulator card and one low-res cards. There was another card with two BNCs
and a switch, could be a grpaihcs card, too. There was a winchester
controller as well, and some card which I couldn't identify.
If anyone's really interested, I'll see how much he wants (probably very
little, they're sold as scrap).
--
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, port 42512.
In a message dated 10/6/01 3:35:56 PM Pacific Daylight Time,
ms(a)silke.rt.schwaben.de writes:
> call "mu-Metal", which is a special alloy of steel and other things i
> have long
> forgotten but that make it EXPENSIVE.
> It is magnetic, but not a good conductor, which make it ideal for
> shielding magnetic
> fields
Mu metal should be easily scroungable. It is the shielding on old large
graphic monitors, Convergent "N"gens and many other pieces of electronic
gear. Often found around CRTs and CPUs. Tektronix used a lot of it.
It is soft and usually a dull gray although sometimes silvery. Occasionally
it shows a crystal type surface. It is magnetic and not a good conductor.
It is expensive new. We got up to $5 per pound for it, scrap, at one time.
Paxton
Astoria, OR
On another list someone mentioned that a PDP11 ran at about the same
speed as a 486-66.
I don't have a 486-66 but do have a PDP11/34 in the garage so I
suggested that if a 486-66 could be found we could have a race.
I tested the capacitors on the weekend and they were OK, still need
to test the RL01 drives and get them hooked up.
>From the cables in the cabinet and the terminator on one of the drives
I'm guessing that they are daisy chained.
A few questions remain.
What would be the best way to test the Power supply it is currently
off the chassis, does it need to be connected for testing?
Any suggestions for a fair race?
Recent additions: Unisys PW2 (Unisys ICON NETWORK), Microcom (apple
clone), SHARP PC4500
Collector of Vintage Computers (www.ncf.ca/~ba600)
Hi,
I'm building myself a 6502-based single-board computer, with the aim of
running it off a 4.8V NiMH or NiCad battery pack. But I've got a problem. I
need a programming language for it. Aparrently there were many BASIC
interpreters sold for the Synertek SYM-1 (aka SY-VIM-1), MOS KIM-1 and
Rockwell AIM-65. Anyone know what happened to the companies that produced
these? Ideally I'd like the source code for a 6502 BASIC interpreter, but
binary code would be OK too. The SBC will communicate via a small LCD panel,
RS232 (so I can hook it up to a "dumb terminal") and will have a hexadecimal
keypad and an optional hard drive, too. Anyone care to make any suggestions
(or submit some code)?
BTW, I also need some info on the Intel 8271 disk controller - I've got two
of these little devils and Intel's datasheet makes absolutely no sense at
all. I've also got a Western Digital WD1770-PH 00-02 that I might be able to
use instead. First of all, which is the better controller? The 8271 or the
1770? Also, is it possible to overclock a 1770 or an 8271 like you can a
1772?
I'd love some schematics and/or some source code. I really don't fancy
disassembling Acorn's 8271 DFS and Watford Electronics' 1770 DFS...
Thanks.
--
Phil.
philpem(a)bigfoot.com
http://www.philpem.f9.co.uk/
We've had the ongoing debate of "build a ZX-81 kit or leave it in the box",
but one aspect of classic kit building I have a question for the group
about is, what about stuff that we built ourselves 10+ years ago that
still works, but might need some touch-up. Specifically, I have several
things (a TVT-6, a Cosmac Elf, a homemade arcade button panel for playing
Space Invaders on a PET, a 2-char LED display for the PET, etc.) that
I made when I was a teenager. They still work, so it's not a question of
repair. It's an aesthetic thing - my soldering skills were much poorer
when I was 13. I'm debating re-soldering these items (and risking breaking
them) or leaving them the way they are a) because if it works, don't
screw with it, and b) it's a snapshot of my own context in the greater
historical framework.
I took history and archaeology in school; I have a strong aversion to
modernizing artifacts. When restoring pots, statues, mosaics, etc.,
an archaeologist tries to never restore an antiquity in a way that can't
be reversed (they use water-soluable glue made from fish scales to move
mosaic fragments, for example). Also, when modern materials are used, no
attempt to make it resemble the ancient material is made - quite the
opposite - it's plainly and intentionally modern looking so ever a
casual observer can't mistake it for the missing original. When my
advisor restored the bed of a large monochrome mosaic near Isthmia, he
threw several modern coins into the concrete bed so that future
excavators would know that it wasn't ancient concrete.
So re-soldering my TVT-6 makes it look nice, but dilutes the fact that
it was originally built in the 1970s, just as much as using modern
ICs does. Do people have an opinion about this? Am I just stuck on
the horns of a false dilemma?
-ethan
__________________________________________________
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>4th attempt: Removed all tinfoil, started up with the lid open, no wobble.
>The wobble appears progressively as the monitor approaches the transformer
>(the lid down position).
Knowing NOTHING about the unit you are working on... two things pop in my
mind from my TV/VCR repair days.
1: Is it possible the controller on the back of the tube is loose? When
you tilt the lid (and from what I gather, the tube with it) back into
place, maybe the board is shifting from the change in weight position.
2: Is there a cable or wire that is getting pulled on when you close the
lid? Maybe one of them has a bad solder joint, and the joint stays
properly closed when the lid is open, but when closed, gets pulled (or
pushed), and breaks the connection. Have you tried, while the unit is
open, to jiggle any wires that might be effected (obviously, don't do
them with your hands, use a non conductive spudger or similar tool...
wouldn't want you to kill yourself by grabbing the wrong wire)
Again, I know NOTHING about the unit you are working on, so these were
just shots in the dark thinking maybe something will help.
-chris
<http://www.mythtech.net>
On October 6, LFessen106(a)aol.com wrote:
> Hey Dave, can you hold onto a Quadra for me?
Sure, if you can grab it before I move...end of October...
-Dave
--
Dave McGuire
Laurel, MD
What you need is Mumetal which is a MAGNETIC alloy.
Lacking that distance helps.
Having built numrous O'scopes over the years the problem of
transformers radiating magnetic fields are well known to me.
Allison
-----Original Message-----
From: Adrian Vickers <avickers(a)solutionengineers.com>
To: classiccmp(a)classiccmp.org <classiccmp(a)classiccmp.org>
Date: Saturday, October 06, 2001 3:39 PM
Subject: Re: CBM8032 - wobbly screen (The saga continues)
>Curiouser and curiouser...
>
>Today, I happened to have the machine opened up to effect some keyboard
>repairs (i.e. a jolly good clean up). Having successfuly removed all
traces
>of key bounce, non-working keys, etc., and feeling justifiably please
with
>myself, it occurred to me to try a couple of things WRT the wobbly
screen.
>
>1st attempt: "build" some shielding out of aluminium foil to go around
the
>transformer. Which I did - all rough & ready stuff, with just a bit of
>insulating tape over the transformer connections to make sure they
didn't
>short out. No effect - the screen wobbled its way along as ususal.
>
>2nd attempt: Same as above, but this time I made absolutely sure the tin
>foil was earthed, by screwing it onto the chassis via one of the
>transformer screws. Same effect as 1st attempt: FA.
>
>3rd attempt: Getting desparate now, I wrapped a load of tin foil around
the
>signal cables (having removed it from the transformer). Decided to leave
>the lid up when I switched on and bingo! Steady as a rock (literally no
>movement whatsoever. Refusing to get too excited, I carefully brought
the
>lid down (holding the signal cables so as not to let the tin foil touch
the
>circuit board - bah! The wobble returned.
>
>4th attempt: Removed all tinfoil, started up with the lid open, no
wobble.
>The wobble appears progressively as the monitor approaches the
transformer
>(the lid down position).
>
>5th attempt: Put a sheet of tinfoil over the whole of the bottom of the
>monitor section. No effect - wobble remained.
>
>Clearly, therefore, the wobble is induced by the monitor. There must be
a
>fairly monster field coming off the transformer interfering with the
>electron gun aiming circuit(s), or something like that.
>
>The question is: What *would* shield it? Do I need some thin steel or
>copper, for example? Presumably, the shielding needs to be earthed (not
>difficult, there's plenty of earth points around there. Or, would it be
>easier to simply replace the big transformer with a smaller modern one?
>
>TIA!
>--
>Cheers, Ade.
>Be where it's at, B-Racing!
>http://b-racing.co.uk
>
