I have an IMSAI 8080. I
checked the address lines and I also get 3.43V. Using 8T97
chips.
You are right about the LS chips not being an exact replacement
for the standard chips.
They will work well enough as drop in replacements
for digital circuits, but a crystal oscillator is an analog circuit.
I have
seen such oscillators fail with same part numbers by different
manufacturers.
That same oscillator circuit even failed by simply
installing a socket. Trust me, I didn't believe it at first but after
installing and removing it a few times, I recognize that crystal oscillators
using TTL are very sensitive.
Lee Hart has taught me that CMOS chips make
much better choices for crystal oscillators.
Back to your Altair.
The 3.4V should work fine, it does on my system.
The ring looks big, but I
don't believe it's beyond a reasonable amount for a functioning system.
You
can probably reduce the ring by terminating that line, but it's quite likely
you don't need to.
May I recommend a "NOP" test? Remove all
boards except the CPU. Pull the DI0-7 lines low. Ground them or
use 100 ohm. There shouldn't be anything driving those DI lines, so be
suspicious if you read voltage on those lines.
This is the NOP command, let
the CPU run, now scope all the lines. Look for pSync, o1, o2, clock,
MEMR, DBIN. The address lines should be counting. Check your CPU
for correct voltage, check the WAIT, HOLD and RESET signals. Your CPU
must be running, if not, then you have a bad CPU chip. If anything isn't
right, trace the signals back to the front panel switches. Check that
DI0-7 is reaching the CPU through the buffer chip.
I hope that idea
helps. good luck.
> From:
er...@osmancrew.com> To:
n8vem...@googlegroups.com>
Subject: RE: [N8VEM-S100:1447] New/Old Project (Altair restoration)
>
Date: Sat, 23 Feb 2013 20:07:43 -0800
>
>
> With advice
from Tom Lafleur and others I've been making some progress on my
>
Altair restoration. This machine was killed by a lightning induced
power
> surge many years ago. I've installed new, modern power supplies,
and I've
> removed all the chips from the front panel and the CPU. These
were replaced
> with sockets. I learned that apparently, you can't
always replace 74xx with
> 74LSxx. I learned this when the system clock
would not run when I tried two
> different 74LS04 chips in the clock
circuit but works fine with a 7404.
>
> Anyway, after
repopulating the chip sockets with mostly LS parts, the Altair
>
exhibits very strange and unstable behavior. I almost don't know where
to
> begin to describe it. Let's just say that the results of a reset
are fairly
> random.
>
> I thought I'd ask a few specific
questions:
> If I "stop" the Altair and check the voltage of an address
line on the bus
> that is high, I get about 3.4 volts. The only boards
in the system are the
> front panel and the CPU. In this configuration,
all that's going on is the
> 8080 address lines are buffered through
74LS367s onto the bus. The front
> panel is involved only insofar as the
address lines each go through an LED
> and a 220 ohm resistor to ground
to display the address line's state. Hard
> to imagine a simpler
situation. The output from the 8080 is a healthy 4.9V
> where it goes
into the 74LS367, but on the output side it is only 3.4V.
> This low
voltage value for a high logic state seems like a potential problem
> to
me. Am I right?
>
> Lest you think it might be a bad 74LS367, be
aware that I previously had the
> functionally equivalent 8T97 chips in
there and had essentially the same
> result.
>
> Also, the
CPU and front panel regulators were replaced and I get a healthy
> 4.96V
on the +5 side of the regulators.
>
> So the immediate questions
are:
> 1. Am I right in saying that the 3.4V level is an issue?
>
2. If so, any ideas what could be causing this?
> 3. I'm attaching the
oscilloscope trace of the system clock as seen on bus
> line 49. Does
that look OK, or is there too much ringing?
>
> CPU Schematic
here:
>
http://www.s100computers.com/Hardware%20Manuals/MITS/8080%20CPU%20Board%20Sc>
hematic.pdf
>
> Thanks for your ideas.
>
> - Eric
Osman
>
> -----Original Message-----
> From:
n8vem...@googlegroups.com [
mailto:n8vem...@googlegroups.com]On
>
Behalf Of Eric Osman
> Sent: Saturday, February 02, 2013 12:48
PM
> To:
n8vem...@googlegroups.com>
Subject: RE: [N8VEM-S100:1357] New/Old Project
>
>
>
Douglas -
>
> Thanks for the introduction and summary. I'm on
Andrew's list for an
> extender, and I'll probably be looking to obtain
a prototyping card as well.
> These are driven by my initial goal, which
is just to get my Altair working
> again. Longer term I'll be looking to
enhance it a bit.
>
> I was intrigued enough by the Raspberry Pi
to get one, but of course, that
> is not the focus of this
board.
>
> I'll be looking over the Wiki's and mail group
archives that you mentioned.
>
> Thanks again.
>
> -
Eric
>
> -----Original Message-----
> From:
n8vem...@googlegroups.com [
mailto:n8vem...@googlegroups.com]On
>
Behalf Of Douglas Goodall
> Sent: Saturday, February 02, 2013 10:52
AM
> To:
n8vem...@googlegroups.com>
Subject: Re: [N8VEM-S100:1355] New/Old Project
>
>
>
Eric,
>
> Our group consists of lots of old-timers and youth as
well.
>
> Apparently all of us like to fiddle with hardware, and
there is quite a lot
> of fun going on.
>
> The major
interests within the group are focused on a range of hardware, as
>
simple as a single
> board computer (See the Zeta), and more
sophisticated buss oriented systems,
> both S-100 and ECB.
>
> Andrew supplies bare circuit boards for us, mini boards, SBC's, and a
highly
> integrated machine we
> started calling the N8
(originally named "Home Computer").
>
> John sells S-100 bare
boards, CPU cards, memory boards, ...
>
> There are two main
Google mail groups, one for Andrew's focus
> (
n8...@googlegroups.com) and
one
> for John's (
n8vem...@googlegroups.com).
There is another one recently
> formed for the scsi to ide
>
project, aka S2I.
>
> Information about the boards, schematics,
board layouts, etc are found on
> the wiki (
n8vem-sbc.pbworks.com).
>
> Building these boards is a learning experience, and we gain knowledge
about
> sourcing parts, building up
> boards and then debug them.
The community members are very happy to help
> each other get things
working
> the google groups are a constant stream of questions and
answers about
> aspects of the hardware and
> software.
>
> There are a number of different BIOSs written by community members,
some of
> which are more specific and
> some of which are more
productized and full featured. If you want to find
> out more about the
boards, look
> under board information on the wiki. There is a software
information section
> as well.
>
> Welcome to our
community, and don't be shy to communicate with us via the
> lists or
privately.
>
> Regards,
>
> Douglas Goodall
>
> On Feb 2, 2013, at 1:23 AM, Eric O <
ewok...@gmail.com> wrote:
>
> > Andrew Lynch suggested I join this group and seek assistance
with my
> "project".
> >
> > Background:
> >
Back in 1975 I was a 20 year-old electrical engineering college
student
> and electronics hobbyist and saw the famous Popular
Electronics article on
> the Altair 8800 computer. I ordered it,
assembled it and it worked great as
> soon as I powered it on for the
first time. Over the next year or three I
> enhanced it with some
additional memory, a homebrew parallel and serial
> interface and the
Processor Technology video card. I wrote hand-assembled
> machine code
to "boot load" my own little monitor via a modem to the
> mainframe
computer on campus. This involved an automated log-in to my
> account,
starting the listing of a hex file and then capturing and loading
> that
hex file into the Altair RAM. Of course I had to switch a couple
>
hundred bytes of machine code into the Altair whenever I needed to
"reboot".
> I also wrote a terminal emulation program so I could then
use it as a
> terminal to that same mainframe. Great fun and done on a
shoe string
> because I was a very poor college student.
>
>
> > Disaster literally struck out of the sky one day around 1979
when a very
> powerful thunderstorm hit and a lightning bolt literally
blew the top off
> the power pole that fed the off-campus house I shared
with three other
> students. I should have unplugged the Altair when the
thunderstorm arrived,
> but I didn't want to have to take 15 minutes to
reboot it. Stupid! Anyway
> the power surge killed the machine. It would
still light up but it wouldn't
> do anything approaching normal
operation. I did replace a number of the
> chips in the weeks that
followed, but I couldn't afford to do a proper job
> of it.
>
>
> > Well, graduation came, then a job, then an IBM 5150, and
then other
> computers over the decades and now the Altair has been
stored in a box for
> almost 35 years. I always meant to fix it someday
but never got around to
> it. But now that I'm semi-retired from a
career in computers I'm finally
> getting around to it. So a couple
months ago I finally got it out of that
> box and started doing a bit of
research and I'm so happy to see all the love
> that people have for
these old machines.
> >
> > One of the first things I
learned was not to trust the original power
> supply. So I went out and
got a couple switching power supplies from
> MeanWell, mounted them up
in the chassis, and leaving the old supply
> physically in place,
removed it electrically and replaced it with the new
> supply.
>
>
> > I popped out all the boards, and turned it on. I'm getting
all the proper
> voltages in all the proper places, including regulated
+5.13 on the display
> board. With the CPU in I get the proper regulated
voltages on the CPU card:
> (-5.25 on Pin 11, +11.69 on pin 28, +5.00
out of the regulator).
> >
> > I've started working on the
front panel and I've already identified two
> inverters with the same
logic state on each side of the gate, on two
> different chips. So I
know I need to replace those.
> >
> > Any and all
suggestions welcome.
> >
> > (I've seen the very good
article at
>
http://s100computers.com/My%20System%20Pages/Debugging/Debugging%20for%20beg>
inners.htm)
> >
> > Eric O
> >
> >
--
> > You received this message because you are subscribed to the
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> > To unsubscribe from
this group and stop receiving emails from it, send an
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>
> >
>
> ---
> Douglas Goodall,
http://goodall.com>
> Note: I
don't use messenger, or skype, or facebook, chat programs in
> general.
Having always-on open communication links through massive public
>
servers I don't have control over seems like too much of an invitation to
be
> infected by a virus or bot. It is bad enough that my Mac wants to
stay in
> periodic contact with Apple's cloud. Skype was tempting before
Microsoft
> bought them. There have been too many examples of remote
session links being
> abused by vendor employees. Even "back to my mac"
makes me nervous. There
> was a recent episode where Apple cooperated
with a social engineer and
> compromised someone's entire electronic
persona. If you want to speak with
> me, calling me on the phone works
well, and you don't have to wonder if the
> electronic mail got through
or not. When I say "Hello, this is Doug", you
> know who you are talking
to. Just in case you were curious.
>
> --
> You received
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