Hi John, Eric, I think John's right. The dipswitch had too much oxidation on the contacts, so it's probably a system wide problem. My Ohio Scientific Superboard had the same problem, I replaced at least 1/2 the sockets before it started working. Looking back, now I wish I had changed them all, because when the others fail, I may not know which sockets are done already. I hope you have a decent temperature controlled soldering iron and a good solder sucker. I always recommend the Soldapult DS-017, full length. It would be even better if you had a full desoldering station, I picked up a 2nd hand PACE unit with a hollow tip and vacuum for cheap. A solder pot is probably the fastest, I wish I had one of them. Cheers, Josh
From: mon...@vitasoft.org To: n8vem...@googlegroups.com Subject: RE: [N8VEM-S100:1506] New/Old Project (Altair restoration) Date: Mon, 11 Mar 2013 16:45:05 -0700 FWIW, I don’t think bus noise is your problem. At low speeds like 2-4MHz you will occasionally have a system randomly hang from bus noise, but if you have consistent problem it’s usually a connection, socket a bad IC or an incorrect jumper/setting. At these kind of speeds, 74XX or 74LSxx are fine. From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Eric Osman
Sent: Monday, March 11, 2013 4:23 PM
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1505] New/Old Project (Altair restoration) Hey, thanks for all the useful code! I'm a little closer to being able to make use of it. I've been working on the SSM-IO4 card and can now output to parallel port A. The DIP switch for the parallel port address is not working correctly. 5 of 7 switches don't make contact when "closed". It took me a while to figure that one out. Who would have thought that those switches would go bad? But, I guess they have contacts inside that are subject to corrosion. Probably, the board wasn't stored properly at some point. There are other signs of poor storage, as well, such as dirt. Regarding my CPU card, I read somewhere of someone who used a function generator to slow down an Altair CPU, but I think I'll wait a bit before trying that. Right now, I think my biggest issue is bus noise, so I'm going to try the N8VEM S-100 Terminator/prototype card when it comes. I've been reading through an extended discussion of bus issues on the retrotechnology web site: The most important point to me is this: "Lee Hart: However, I'm not sure that propagation delays are the issue for bus noise. What I see happening is that the S-100 bus was designed for old slow TTL, and isn't suited for modern high-speed logic. If you just substitute faster chips (74LS --> 74S --> 74ABT etc.), the speed of the rising and falling edges are so sharp that they generate lots more ringing and noise. And, the faster chips are capable of *responding* to this noise as if it were signals. " When I replaced the chips on the CPU and front panel, I used almost all LS or ALS parts, usually in place of regular old 74xx chips, and I suspect that these faster chips are causing additional bus noise. In the above discussion, Lee goes on to suggest 74HC types as they have softer rise/fall times. There is also a discussion of how the worst ever was the Altair bus with two 4 slot backplanes wired together, plus a bunch of long wires going to the front panel. Guess what? That's exactly what I have. But I'd prefer to keep it that way if possible, for the sake of keeping the system as "original" as practical. I do have another mobo I can put in there if worse comes to worse. HI Eric,
It's only 16 bytes so it's very simple.
;16 BYTE BOOT STRAP TO LOAD PROGRAM FROM SERIAL PORT.
0000 211000 LXI HL,$0010 ;First free location
0003 DB00 IN 00 ;GET UART STATUS
0005 07 RLC ;SHIFT DAV TO CARRY FLAG
0006 DA0300 JC $0003 ;LOOP BACK UNTIL RXCHAR
0009 DB01 IN 01 ;GET CHAR
000B 77 MOV M,A ;SAVE IN MEMORY
000C 23 INX HL ;ADVANCE MEMORY
000D C30300 JMP $0003 ;LOOP BACK FOR NEXT CHAR
After you toggle it in, you can reset and run, then have your PC send binary characters to be stored at address $0010 and up.
I was using a small VB program I wrote that just opened a binary file and sent the bytes. There's no error checking, but it worked well enough to get a basic monitor into RAM. I also wrote up a decent monitor that includes XModem. I'll send this as an attachment. You will need to assemble it. You can delete the Floppy Disk section, since that will only work with the Tarbell MDL 1011 controller (or any other WD 1771 based FDD controller).
I am assuming in this code that your SSM IO-4 is strapped for Altair Mode (this was how mine is/was strapped when I got the IMSAI).
After the file transfer is complete, reset the computer, replace the first byte (21) with C3 to cause a Jump to $0010, then flip the run switch. If you want to try loading my Monitor program in, then add something like:
.ORG 0010h
JMP MAINMENU
Also, check where "MAINPROGRAM" equates to, this has to point to RAM space.
That is so strange that all those 8080A's aren't working for all codes... I am thinking something is still wrong with your CPU and/or FP boards.
Perhaps try slowing down your clock? You might be able to just drop a different crystal into the 8224 circuit. It's expecting a series cut crystal, my knowledge ends here, so I can't say if a parallel cut crystal would work, my guess is there's no harm in trying. Toss in a 32Khz crystal, that should make for a nice and slow system, then you can definitely scope for bus noise, or contention.
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1495] New/Old Project (Altair restoration)
Date: Sat, 9 Mar 2013 10:26:18 -0800 If that yellow jack on the new TVs will work, then I should be fine, as we have a small one that isn't being used, except occasionally in the guest room. I'd be interested in your 16 byte bootstrap, if you're willing to share. My old bootstrap was considerably longer and I'll need to do something along those lines until I get some kind of ROM board going. Eventually, I'd like to get to the point where it us using a virtual floppy or hard drive on flash. I took a couple hours out last night to test four old 8080 chips I had lying around. The one I've been using is a recent purchase from Jameco, an actual Intel part, amazingly enough. Anyway, they all "kind of" work. In other words, they will properly execute some instructions and not others. I found this surprising. I would have thought they would either not work at all, or work 100%. Eric,
I totally understand time restrictions on these projects. I thought it would take me 4 months to get my IMSAI running, but between family, work and other projects it took a solid 13 months and there's still more I'd like to do. It's like every new idea spawns two more. Even interest in this hobby waxes and wanes.
I'm aware that new TV's no longer have the analog tuner but most if not all of the new TV's I've seen still have a Yellow RCA jack for plain NTSC video input. But, you probably don't want to use your main TV, instead, try to sit your wife down in front of it with a chick flick while you tinker with the Altair. For this purpose, it's not a bad idea to find an older TV, something more retro to match the Altair. Or you can also find security monitors, they all take standard video in. Check your local craigslist listing, I'm sure you'll find many bargains. I also see many old TV's for $10 or less at garage sales.
PS. I also have a SSM IO-4 card. Getting your system to boot up will require a monitor in ROM, or some kind of floppy disk system. The ROM card is cheaper, Rich Cini uses a similar approach, if you search the forum or his site you can find more info on the RAM/ROM card he's using. It's a very cool board, because it lets you pop out the 2K RAM chip and replace it with a 2K ROM chip for that part of the memory map.
Before I got my system to boot, I would toggle in a 16 byte boot strap to load programs from the PC. It's fun to do the first dozen times, then it gets old and you start to wonder how much wear and tear you want to put on your switches.
Regards,
Josh
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1491] New/Old Project (Altair restoration)
Date: Wed, 6 Mar 2013 21:17:22 -0800 Your suggestion to clean the edge connectors and slots is a good one and I will be doing that soon. I'm going to have to reduce the time I spend on this project for a while so I can get my taxes done and do some other things to keep the wife happy. But I still plan to try to put 3 or 4 hours a week into it. Here's a question: The Processor Technology VDM-1 puts out composite video. My old TV with composite video input is long gone. What do people recommend for converting composite video into something a modern monitor can use? That SBC you're putting together looks pretty ambitious. It will be interesting to see the circuit diagrams when you get done. Maybe I'll build a copy one of these days. But first, of course, I need to finish getting all my current hardware working, which will be quite a project in itself: o Another Altair CPU card o Two Processor Tech 4K SRAM cards o A processor Technology VDM-1 o A Thinkertoys 32K SRAM card o A Solid State Music IO-4 card Then there's the matter of setting up some way of booting something useful, even just a monitor, with a minimum of switch flipping. I'd forgotten how tedious and error prone that is. Hey Eric,
You made a good point about testing the RAM for hours. I only ran my tests for a few loops, not more than 30 minutes, but I should be testing it like you are doing for a solid 24+ hours.
I find the edge card connectors need a good scrubbing, and you gotta get into the S-100 edge card sockets too. I wrote about this recently, if it was to you, then forgive me for repeating. To get into the sockets, cut one of those white rubber erasers into 1/16" thick slices and press it in and out of the socket, be sure to brush, vacuum and blow the sockets clean afterward.
This is great that you've got a good point to start from. It can be very frustrating when everything that can go wrong has gone wrong.
I had a weird (conditional) problem on my I/O card, but lost my patience and replaced 6 possible chips. I fixed the problem but didn't learn anything in the process.
I just recently powered up a home brew 8080 card for another IMSAI I'm working on. The cpu didn't work right away, there were issues. FIrst it was the ready line that wasn't Active due to an I/O interface latch, next I found that I didn't wire the HOLD and INT lines, so they did go Active! Then another chip didn't have VCC connected, got that working but the CPU control (Examine/Deposit) didn't work (added some more logic to disable the RAM while operating the CPU). Lots of little problems, but I can't tear myself away from working on it, too damn addictive! Look for another post to Andrew for more details on this card if you want to see pictures.
Cheers,
Josh
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1488] New/Old Project (Altair restoration)
Date: Tue, 5 Mar 2013 21:05:42 -0800 Well that 32K RAM card didn't work out as well as I hoped. It failed a memory test sporadically so I decided to concentrate for now on my good, old 1K RAM card from MITS. After tracking down several problems with the support logic, I just went ahead and replaced it all. Then I wound up replacing one of the RAM chips. When running diagnostics it would stop with a dropped bit after anywhere from 2 to 6 hours and this would occur on any of the RAM chips, including the one I replaced. Always bit zero. I was running this diagnostic on the 1K card with the 32K card still in the system. I decided to remove the 32K RAM card and move the 1K card right next to the CPU. It has now run over 26 hours with no dropped bits, although it has stopped twice when the memory protect turned on by itself. So I'm thinking I have a noisy bus and will need to do something about that before I can make more progress toward a reliable system. I have a terminated bus on the way, but it will be a few days. Then I'll have to use a proto card to hook the front panel into the bus. All 70 wires, approximately. Yuk. I hear what you're saying about the older memory cards being unreliable. I hope I'm able to get mine working because I'd like to keep the system as close to "original" as possible. Of course swapping out the bus isn't exactly original, but that old Altair bus really sucks. Alternatively, I could try adding a terminator card to the end of the Altair bus. Once I have a reliable system, even with just 1K of RAM, I'll have a known good base from which to start trying to fix the other cards. Thanks for all your help. Hi Eric,
Excellent work!
Yes, you are right about the RESET line needing to be low, sorry that was my mistake. I thought it was a "NOT" RESET line, that's why I put an asterix after the name RESET*. FYI, In the old S-100 bus descriptions, an asterix indicates a "NOT" logic pulse, eg Pin 77 is pWR* for "NOT" Write.
My IMSAI also has a noisy o1 and o2, see picture attached.
Perhaps you can improve your wave forms by adding some decoupling capacitors near the clock?
You might have dirty edge card connections. Get a nice white polymer eraser and give the pcboard fingers a good scrubbing. I even slice the eraser into pcboard thickness and press in to the connectors. Just be sure to fish or blow out all the rub-off.
On the IMSAI, I tried several memory boards, they all were flakey. I had to build a new RAM card with modern chips. Later RAM tests confirmed my suspicions on those old memory boards.
But, overall it sounds like you are well on your way to fix everything. Looking Good!
Regards,
Josh
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1476] New/Old Project (Altair restoration)
Date: Wed, 27 Feb 2013 19:51:09 -0800 I went through all the checks you suggested and everything looked good. The only exception is that I found reset on pin 12 to be low instead of high. This is what I would expect based on my reading. Was that a typo, or am I missing something? Some of the waveforms were a bit ratty, but OK, I guess. I don't want to clutter everyone's inboxes with a bunch of pictures, so I'm only attaching one. It is of the two clock phases on pins 15 and 22 of the CPU chip. This was a real eye opener to me a relative neophyte. Not the pretty square waves they show in the book, that's for sure. After replacing the one 8T97 and getting much better voltages on the bus associated with those lines I went ahead and replaced all of them all and viola, I have a CPU board that seems to work! The front panel functions seem to work as well. I plugged in a recently acquired 32K static RAM board and everything seemed to work except that I could deposit to memory using the front panel, but the CPU did not seem to be able to write to memory. However, it started working when I plugged the memory card into a different slot. Spooky. I haven't yet put it back in the original slot to see what happens but this seems like a problem I'd better get to the bottom of. I also have another CPU card and a few RAM cards to get working. Thanks again for all your help. Hi Eric,
That does sound like good news. Those 8T97's on the 2nd CPU board might be the issue there.
Sockets on all the chips shouldn't make a big difference, just as long as they are decent sockets.
The only very sensitive chip that you might not want to socket is the Crystal Oscillator chip, 7404.
How's your soldering? did you give a good visual for solder shorts? I like to hold the board up with a bright light behind so the board glows, you can see solder shorts and/or micro shorts.
That's wonderful that you have a 2nd board. Now you can compare signal pulses between them. Go pin by pin on the CPU board, make your notes or take pictures.
Yes, it does eat lots of time. I have the same frustration with time. Every project turns into hours, but I love doing it so those hours pass by so fast! But rest assured, you will find the problem(s) and fix if you keep studying the schematic, datasheets, etc The victory in the end is worth it! :)
Cheers,
Josh
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1471] New/Old Project (Altair restoration)
Date: Mon, 25 Feb 2013 11:59:49 -0800 Thanks for all the pointers and ideas. Voltage levels to CPU chip OK. I wired up a proto board to pull the DI lines low for the NOP test as you suggested. Going through a 100 ohm resistor didn't seem to successfully pull the lines low, so I tied directly to ground. I still observe very flaky behavior with that CPU board that I can't begin to characterize. You've got me wondering if those sockets I put on that board were a good idea. I have another used Altair CPU board I bought a few years ago in anticipation of this project. When I tried it the address bus does "count" as you suggest, but that board has its own problems. I can stop the processor and reset. Examine next seems to work, if I look real close at the lights. This got me to checking the voltage levels on the address bus (with the processor stopped). "High" is in the range of 1V. "Low" is in the range of 0.1V. This varies a bit from one address line to the next. For some lines "high" is in the range of 1.5V which is enough to actually light the LED on the front panel. Recall that with the first CPU card (the one I rebuilt) I was seeing more like 3.4V. I verified that the addresses output to the 8T97 from the CPU chip are in the +5V range. I notice the 8T97 seems to be getting fairly warm so it must be dissipating all that voltage internally for some reason. My current plan is to pull one of the 8T97s from this CPU and put a socket in there to see if a new 8T97 works better. If I can get a known working CPU I feel like I can begin to debug any problems I may have on the front panel. Fun stuff. Eating lots of time. Thanks again. I really appreciate your help. Hi Eric,
Yes, you must have the front panel in place. It is required to put the CPU in a "RUN" mode. The front panel controls or generates the CPU mode (RUN, WAIT or RESET). The 2 lines of interest are the pRESET (pin 75 on the S-100) and XRDY (pin 3).
Other S-100 lines of interest are pRDY (pin 72), and pHOLD (pin 74).
When running the NOP test, you want to scope every pin on the CPU to understand what it's doing.
Look at the timing diagrams of the 8080A data sheet.
1st check the voltage pins (good idea to check ground pin 2 with ohm meter prior to powering system).
2nd check the CPU clocks, Pin 22 & 15 should have non-overlapping positive pulses to 12V. These are not TTL inputs.
3rd check the CPU mode, Reset* pin 12 must be high, Ready pin 23 must be high, Hold pin 13 must be low, INT pin 14 should be low.
The mode inputs should be steady, no pulses.
4th check the CPU timing output pulses. WR* pin 18 should be high (we're executing NOP's, there should be no memory writes), DBIN should be pulsing high to fetch/read the NOP instructions, Sync pin 19 should be pulsing high during M1 of every cycle as per the data sheet.
5th Scope your data lines, these should be LOW (as tied low for the NOP) during the DBIN pulses, some pins should pulse high during the PSYNC to indicate M1 and sMEMR cycles (read the data sheet).
6th Scope your address lines, these should be counting up, you should see them all toggling high/low. Follow this signal onto the S-100 bus.
The advantage of running NOP's, is that the CPU wave forms are repetitive and easy to follow on the scope.
Also, it eliminates all other memory and I/O boards, breaks your system down to the basics.
See CPU run.
Run CPU run.
Once you can get this far, it's likely that your system will operate if the other boards are good.
But getting here isn't always so easy.
I did a similar test to fix my OHIO Superboard. It was working intermittently with the NOP test. The problem was bad sockets, to make things a challenge, when I would scope a pin, the pressure put on that pin would make it temporarily connect. I think the Altair used good quality sockets, but best to keep your wits about you!
Please keep in mind that I'm running an IMSAI 8080, but I think it's almost an identical copy of the Altair.
Cheers,
Josh
From: er...@osmancrew.com
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1452] New/Old Project (Altair restoration)
Date: Sat, 23 Feb 2013 21:54:57 -0800 I assume I can leave the front panel hooked up for the NOP test? I dread the thought of removing and replacing all those wires again. I actually have three different 8080 chips and the computer doesn't work with any of them, although the symptoms change slightly. In any case I have a new one on order "just in case". It is true that none of the three are "known good", so I like the idea of this test, I just hope I can do it with the front panel in place. Hi Eric,
Very cool!
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.
Josh
> 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 74LS367’s 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
> >
> > --
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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.
>
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