Altair 8800c build thread

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Re: Altair 8800c build thread

Postby Curto » April 20th, 2022, 11:01 am

Indeed I do! I am a little dyslexic when I am tired. Good catch, thank you!
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Re: Altair 8800c build thread

Postby Ttpilot » April 22nd, 2022, 12:42 am

Between the front panel and the 8080 board mine has not worked quite right, but for the life of me I haven’t been able to pin it down. Have you adjusted the clock timing yet?
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Re: Altair 8800c build thread

Postby Wayne Parham » April 26th, 2022, 1:06 pm

I had a little time last week to work on the system, and I soldered most of the parts on the backplane(s). Plural, because I have both an 18-slot board and a 9-slot board. I have a terminator card for the 18-slotter and I populated the termination circuitry on the 9-slotter. But heavens knows why, because I intend to run the original 8080 board, so I probably don't need termination anyway.

Well, not the original 8080 CPU board, but the Kaufman processor board. Same thing but with a pretty solder mask. :-)

Altair8800c_9slot_backplane_populated.jpg
Populated 9-slot backplane

You'll notice a couple resistors missing and also the op-amp isn't installed. I'm still waiting on those parts. I wasn't going to socket that chip but it is going to be a while before it gets here so I decided to socket it after all.

You may also notice that the fuses are "socketed." I always hated desoldering those little pico fuses so I installed a header pin on each end. Better than a jumper, and better than a soldered pico fuse. I also soldered in header strips for the terminator resistors. I just wanted to be able to remove terminators, just for fun. Totally didn't need to do any of that, I'm sure.

Nothing interesting to show on the 18-slot backplane so I won't upload a photo. It's nothing but edge connectors. I will make an observation that it is really weird how MITS applied power to that board though. I downloaded the 8800b assembly manual to see how they connected power and, wow, cool. I'll do the same. But I'm doing it with three inline fuse holders. I'll connect those plus ground to a four-position barrier strip, and connect that to the power supplies. And I'll stick to my own little "standard" when connecting the 9-slot backplane - board to barrier strip to power supplies.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 16th, 2022, 4:52 pm

I've done a lot on the build(s) since last post. Just been too busy to update the thread. But here are some of the high points:

I've got a couple of builds going simultaneously. The main reason is just to have spare parts. But it also lets me make two configurations that are slightly different. One has the 18-slot backpanel and the other has the 9-slot board.

I've also experimented a little with the nine-slotter, mostly in respect to the card cage. But I'll post info about that in the "3D Printing Service" thread, which is predominantly about options for getting card cases made for the 9-slot backplane.

As I said, I've made two builds and powered them both up in a minimal configuration. They each have (1) 8080 CPU board, (1) front-panel interface and (1) FDC+ board configured to expose its 64Kb memory. I can examine and modify memory locations and I have entered and run "Kill the Bit."

Altair8800c_red_first_power.jpg
Bare-bones Altair 8800c

I use a pair of four-position barrier strips to distribute power. One is for the AC input side and the other is for the DC outputs from the power supplies. The DC lines are black (gnd), red (+8v), violet (+16v) and blue (-16v). The AC strip has two for line and neutral, straight in from the AC line. The neutral is black and line is white. Black is distributed to the neutral input on all power supplies. White goes out to a panel-mount 3A fuse and comes back to the third AC barrier-strip position. That goes to the power switch connector on the frontpanel, and returns to the fourth AC barrier-strip position. The fourth position is what runs to the line input on all power supplies. I also run the fan off that. Earth ground is green and isn't connected to a barrier strip. It runs to chassis ground and to all the earth-ground connections on each supply.

Now for something that may be seen as pure sacrilege. I installed blue LEDs on the front-panel for the second system.

Altair8800c_blue_first_power.jpg
Altair 8800c front-panel with blue LEDs

It definitely doesn't look original that way but I think it looks awesome. I'm pretty sure if blue LEDs were around when Ed Roberts made the first Altairs, he would have used 'em. I just thought my Altair 8800c - being a "restomod" of sorts - really needed blue front-panel lights.

So the one I'm making with the 18-slot backplane will have a red (original-lookin') front panel and the one with the 9-slot backplane will be sportin' blue.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 16th, 2022, 6:27 pm

Something else I wanted to show you guys.

Let me fold that front-panel interface ribbon cable out of the way to show you the processor:

Altair8800c_Intel_Inside.jpg
"Intel Inside"

Let's zoom in a little bit:

Altair8800c_Intel_Inside_closeup.jpg
Intel 8080A

It's a ceramic 8080A, made by Intel.

The processor chip that came in the CPU parts kit works just fine, but I had to swap-in the ceramic chip I had in my collection, just for fun. It still works.

Hey - question for the group - notice the damage to the ceramic near pin 20. I heard a rumor at one time, probably read it actually, that those were fairly common production defects and that Intel used to give 'em away as samples or perhaps sell them at discount. I also heard - actually read an interview with Ed Roberts - that this rumor started some drama back in the day because his competitors said Intel rejects were going into Altairs to keep the cost down. But that's a different issue. Even if the blem chips weren't going into Altairs, I could see them being given out as samples by Intel. Anyone here know if that's true?

And a second question: Would it decrease the value of that chip if I filled in the damage with an epoxy ceramic repair kit? I could probably match the color very well and make it look almost perfect. Dentists do that sort of thing all the time to repair teeth. But I have never done that because I thought it might decrease the value. Sort of like polishing an old coin - numismatists don't like that 'cause it decreases the value. What say you?
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Re: Altair 8800c build thread

Postby BillO » May 16th, 2022, 7:30 pm

Only thing better than an 8800c is a second 8800c!

I had toyed with the idea of using 4 different colors of LEDs in my build, but at the last minute decided to go down the road most traveled.

I'd leave the chipped chip the way it is. Adds character. And I'd use it for sure. It was meant to be used.
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Re: Altair 8800c build thread

Postby Curto » May 17th, 2022, 5:05 pm

Enjoying watching your build. I am just a few days behind you. I have completed all the board assembly, just need to mount and connect the power supplies then it is power on time. I chose to keep the backplane completely passive. I heard that adding the terminations can cause problems with the front panel and other operation. Your's is working good?

Curt.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 18th, 2022, 10:43 am

I socketed the termination resistors and left them out so far. I set the termination voltage with the trim pot, but it doesn't matter 'cause the resistor banks are all disconnected.

At some point, I may hang a scope on the lines to see what they look like on the 9-slot and the 18-slot backplanes, with and without various forms of termination. Or maybe I won't, since I don't plan to use any other processor board than the original 2Mhz 8080. But I did have that in mind as a possibility, just to see if any configurations were more stable and if any didn't work.

I also have a couple of 9-slot backplanes - one completely bare except connectors and one with termination components mounted. The reason I have both of them is I am working through the card cage issues, finding a couple of "optimized" sets. The ones designed by Walt Perko in the "3D Printing Service" work pretty well for an unpopulated (passive) 9-slot backplane but they don't work if termination components are mounted. Your findings on that thread are what set me on this path.

The Perko card guides also require the backplane to be mounted a little high for my tastes - about 5/8" - and I'm wanting to use a little bit shorter stand-offs, I think. So as I get time, I'm going to measure stuff with a micrometer and find out exactly what size stand-offs are required, and to modify the guide to be suitable for a terminated backplane, possibly using a shorter stand-off. I need to see what kinds of (readily available) hardware is required too. I'll report what I find in the 3D Printing Service thread, and attach a v2 card guide model there too.
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Re: Altair 8800c build thread

Postby toml_12953 » May 18th, 2022, 1:36 pm

Wayne Parham wrote:Something else I wanted to show you guys.

Hey - question for the group - notice the damage to the ceramic near pin 20. I heard a rumor at one time, probably read it actually, that those were fairly common production defects and that Intel used to give 'em away as samples or perhaps sell them at discount. I also heard - actually read an interview with Ed Roberts - that this rumor started some drama back in the day because his competitors said Intel rejects were going into Altairs to keep the cost down. But that's a different issue. Even if the blem chips weren't going into Altairs, I could see them being given out as samples by Intel. Anyone here know if that's true?


Roberts got an incredible deal on cosmetic rejects of the 8080 from Intel. He could sell the entire Altair base unit for less than the cost of the CPU alone in single quantities.
In those days, all you had to do to get free stuff from Intel, National Semiconductor, TI or any chip maker was to tell them you were a college student doing research. There was no verification required.
I did that (I was actually telling the truth) and ended up getting a tray of 24 8080s, some 8008s and a few 4004s plus F8s from Fairchild and a number of dynamic RAM chips from TI.
All the companies sent thick packets of literature (I mean LA phone directory-sized packets!) and business cards of some high-level technical people in their labs that I was invited to call any time.
I spent a few hours on the phone with some kids my own age who had written the Altair BASIC interpreter for MITS when I reported some bugs I had found.
Who knew Gates, Allen and Davidoff would become a bit famous?
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Re: Altair 8800c build thread

Postby Wayne Parham » May 18th, 2022, 2:38 pm

Such cool memories! And I hear you. I'm from that era too, and I got a ton of stuff as samples. But even with samples, I was on a budget that moved me towards 6502-based systems, mostly because there was some really affordable entry-level stuff. My career path moved me into minicomputers, because that's really where all the computer jobs were in those days. The world ran on minicomputers back then. I was an engineer for Data General.

I heard the rumor about Ed Roberts using reject 8080 chips. It wouldn't have been terrible - because cosmetic defects sometimes work perfectly well - but I read an interview where he denied ever using reject chips. He specifically says he didn't use rejects, even "cosmetic" defects. This is the interview I'm talking about:

I was really more curious if Intel ever released chips with cosmetic defects, as samples or as inexpensive "blems." I know they gave samples, 'cause just like you said, everyone did back then. Tell 'em you're an engineer and you want to sample a specific part or chip family, and they would likely send you some samples and manuals. I just didn't know if they gave out the "blems."

I don't recall ever getting a sample in ceramic. Seems like all the samples I got from various manufacturers were chips in plastic packaging. And I don't recall ever getting anything with a cosmetic defect. Never asked for samples from Intel though.

Sounds like what you are saying is that Intel did, in fact, distribute chips with cosmetic defects. I think you're also saying that maybe MITS did use those chips with cosmetic defects. Wouldn't bother me in the least if they did. Just interesting "trivial trivia."
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