--------ORIGINAL MESSAGE:
Date: Mon, 16 Jun 2008 22:04:49 -0400
From: "Roy J. Tellason" <rtellason at verizon.net>
Subject: Re: non-CP/M Z80 board
<snip>
I'm not familiar with many 6800 designs, but I was somewhat surprised to see
how limited some parts are, like the 8085 in the "8085 Cookbook" where you
could really get away with very little. OTOH, the c64 doesn't use any
buffering _at all_ and yet the CPU in there seems to have little trouble
driving 3 ROM chips, a set of 8 4164s, plus all the peripherals. I'm
guessing that the Z80 is probably somewhere in between, and that the
datasheet probalby won't give me the whole story anyhow.
<snip>
--------REPLY:
Don't forget that CPUs like the 8085 need data/address MUX chips anyway,
which are usually also buffers.
m
---------ORIGINAL MESSAGE:
Date: Mon, 16 Jun 2008 22:23:39 +0200
From: "Steve Maddison" <steve at cosam.org>
Subject: Re: Fixing a DEC Letterwriter (LA100)
2008/6/16 M H Stein <dm561 at torfree.net>:
> Various patterns of DSR and POWER/FAULT flashing are the diagnostic
> failure codes; what pattern do you have that is not documented?
>
> Defective RAM is 8 flashes BTW.
Yep, I figured that one out the long way. I only have some user docs
so this wasn't listed, just things like "out of paper" and "cover
open". I'm now back to the original CTS and ONLINE combination (no
flashing).
--------REPLY:
POWER/FAULT not flashing should indicate that it has passed POST;
DSR means what you'd think, that DSR is active.
Looks like it thinks everything's OK.
1 to 9 flashes indicate faults with ROMS 1 to 7 & normal/extended RAM.
Steady flash, no bell: open cover or paper fault.
Steady flash w/bell: head jam or servo motor/encoder
m
non-CP/M Z80 board
Roy J. Tellason rtellason at verizon.net
<mailto:cctalk%40classiccmp.org?Subject=non-CP/M%20Z80%20board&In-Reply-To=>
Mon Jun 16 20:01:54 CDT 2008
* Previous message: Fixing a DEC Letterwriter (LA100)
* Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
________________________________
I'm thinking about building a z80 board, with varying amounts of "stuff"
attached, with RAM and ROM that may be NMOS or CMOS, and some
indeterminate
number of Z80-family and compatible peripheral chips.
My question is this: At what point do you _need_ to have address and data
bus
buffer chips?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies.
--James
M Dakin
-----REPLY-----
Hi,
I think the safe range for 74LSxxx style TTL is about 5-10 or so connections
per output gate.
You can compute the ratio of the various datasheet currents and get a more
exact figure.
However, here is a pretty good set of "rules of thumb" for circuit design:
http://kurser.iha.dk/eit/dtm1/DTM1_html/Wakerly/udgave3/xc03.pdf
See line 3.49 for the table of the various combinations. They say 74LSxxx
on 74LSxxx can do up to 20 but I'd keep it lower to be safe.
That should be enough for some static RAM, a ROM, a couple of peripherals,
some simple glue logic and that's about it.
You can stretch it a bit using CMOS chips or the more recent TTL families as
long as you keep the PCB fairly compact.
Doing anything more than the above or going off board with a bus requires
buffers.
You'll find there are few if any absolute rules on the subject but lots and
lots of opinions.
If you examine the IBM XT motherboard schematic, you'll see it uses many
buffer chips -- primarily due to the DRAM, ISA bus interface, and decent
sized PCB.
On the WaveMate Bullet schematic, it has many buffers to for similar
reasons.
There is only so big you can make the PCB and only so much current can be
sunk before the signals start to degrade.
If you want to avoid buffers, think 1) small PCB, 2) small number of chips,
and 3) shortest wiring connections as possible.
Thanks and good luck with your project!
Andrew Lynch
Date: Mon, 16 Jun 2008 03:54:51 +0000
From: Ethan Dicks <ethan.dicks at usap.gov>
Subject: Re: time to part with my PDP-11 system
<snip>
>...Eventually, I'll do another camping trip to Hamilton...
<snip>
-ethan
---
I'm in Hamilton every week or so, and I have to ask: why?
;-)
mike
-----------ORIGINAL MESSAGE:
Date: Mon, 16 Jun 2008 11:06:36 +0200
From: "Steve Maddison" <steve at cosam.org>
Subject: Re: Fixing a DEC Letterwriter (LA100)
2008/6/14 Tony Duell <ard at p850ug1.demon.co.uk>:
>> Anyway - the original problem is gone. With the (half) new RAM I'm now
>> getting what at least appears to be a proper fault indication on the
>> LEDs: both DSR and POWER/FAULT are flashing. This combination isn't in
>> the user manual so my guess is it's failing the POST. If any of the
>
> Maybe soemthing like 'RAM Error' :-)
That was my first guess too. Running the ROM through a simulator and
forcing the RAM test to fail confirms it. The problem? I'd managed to
gouge out a bit of track while removing the ICs - just so happens it
was one of the very tracks connecting /all/ the RAM to the address
bus, so checking for continuity between the sockets I'd soldered in
didn't reveal the problem.
So I fixed that and robbed a couple more 2114s from another machine.
So all RAM is replaced but, lo and behold, ONLINE and CSR are back
with a vengeance. So I'm basically back to square one, although not
entirely as it would at least appear the RAM is OK. It could of course
still be failing in one of the peculiar ways you mentioned and still
pass the POST... Otherwise it would appear the original LED
combination was directly asserted by the processor, so it should be
traceable in the ROM code.
--
Steve Maddison
http://www.cosam.org/
"The great thing about standards is that there are so many to choose from."
---------REPLY:
Various patterns of DSR and POWER/FAULT flashing are the diagnostic
failure codes; what pattern do you have that is not documented?
Defective RAM is 8 flashes BTW.
You ran the ROM through a simulator??? There's an LA100 simulator?
m
Is there anyone in or close to Acton MA that could pick up some computer terminals for me? Unfortunately, they need to be picked up during business hours M-F, and while I'm willing to make the drive to Acton to pick them up, I can't get there during the week because of work. There's only seven of them, so they'd fit in a car. But, shipping would be prohibitive. So, basically, I want to see if someone would be willing to pick them up and hold them for me until this (or next) weekend, where I could make the drive.
Thanks!
-Ian
---------Original Message:
From: "Steve Maddison" <steve at cosam.org>
Subject: Fixing a DEC Letterwriter (LA100)
Hello all,
I have a half-dead Letterwriter 100 KSR here I'd like to get working
again. I get a short buzz from the paper feed motor at power-up but
instead of flashing all it's indicator LEDs, both CTS and LINE stay
illuminated. There's no carriage movement and the keyboard does
nothing either. The basic vital signs are OK but I'm stuck without a
service manual or schematics. Does anyone have access to these docs?
I'd of course also be interested to hear of any common problems and/or
things to check before I dive into all-out mending mode...
Cheers,
--
Steve Maddison
http://www.cosam.org/
----------Reply:
Can't suggest where to start checking but I do have several RO LA100s
awaiting the trip to the scrapper, in case I can help with any non-kbd parts
before they go.
I do also have the tech manual somewhere; unfortunately IIRC it's on 17"
paper so I can't scan it.
I believe Tony also has a copy and more knowledge/experience than I;
maybe he has some suggestions.
m
I have a PDP-11 system, fairly recent, about the size of a mini-fridge, on wheels.
last I checked it was still fully operational, but I can't speak to that now, it's spent the last 2 years in storage.
I just don't have space for it, and it's a bit larger than I can find use for.
its about 4 feet long, 2 feet wide and perhaps 3 feet high - thats by eye ball.
it has a TK70 tape drive that's in excellent shape.
the plastic cover over the front is missing which is why I'm not sure of the model.
I'd rather sell or trade it, if there's interest.
i'm looking for a vax station 3100 model 76 or a 4000 model 90
or some volker craig terminals.
I'd prefer to avoid junking it.
Keep in mind I'm not interested in shipping it, it weighs a bloody ton!
located in Scarborough, Ontario (Canada), so anyone in the GTA area would be an easier pick up.
Dan.
_________________________________________________________________
If you like crossword puzzles, then you'll love Flexicon, a game which combines four overlapping crossword puzzles into one!
http://g.msn.ca/ca55/208
If this is the Apple I I'm thinking of, it WAS bought at a garage sale for a
couple of bucks. The person selling it said they were going to put it on oBay,
and a friend of his and mine asked me about it. I referred them both to Sellam,
and this is the first I've heard about it since then.
> From: Al Kossow <aek at bitsavers.org>
> Subject: Re: Apple I on ebay
>
> "This computer will only be sold to a MUSEUM OR SERIOUS COLLECTOR ONLY"
>
>
> eBay is absolutely the WRONG venue to try proving that.
>
> The whole notion of someone going to an estate sale and picking this
> up is unfortunate as well. I though Sellam knew where all of them were?
