Altair 8800c build thread

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

Re: Altair 8800c build thread

Postby CodeCage » May 18th, 2022, 3:11 pm

Thinking of adding an IEC connector to my 8800c build to replace the included AC power cord.

What's everyone's thoughts on whether or not this voids the retro look. It would be on the back when everyone was marveling over the switches and LEDs!
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Re: Altair 8800c build thread

Postby Curto » May 19th, 2022, 11:56 pm

I agree 100% the walt perko guides sit too tall. I am going to run them through my table saw and use shorter standoffs.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 21st, 2022, 3:26 pm

Curto wrote:I agree 100% the walt perko guides sit too tall. I am going to run them through my table saw and use shorter standoffs.

I think that's a great idea. Totally do-able.

CodeCage wrote:Thinking of adding an IEC connector to my 8800c build to replace the included AC power cord.

What's everyone's thoughts on whether or not this voids the retro look. It would be on the back when everyone was marveling over the switches and LEDs!

I think it's a great idea too. I see the 8800c as being Mike's improvement on the original 8800, sort of a "restomod," to use a car hobbyist term. I've used those AC panel mount connectors on various prototype (and production) builds and they are handy 'cause you can get power cords anywhere.

But how would you cut the hole? Would you drill a starter hole and then use a nibbler? I tend to like having a machine shop punch holes that aren't round - like DB-25 and Centronics cutouts - and they do a great job on those AC connectors too. But I always have 'em punch on flat panels, like the front and rear panels of a Metcase enclosure.

Maybe I'm over-thinking it. The rear panel of the 8800c case just has the mounting "lip" bent into it, so maybe it's no problem for a machine shop to punch holes in that panel. I think they use a hydraulic press rather than a strike, but I'm not sure. I'm also not sure if the panel needs to be flat for the shop, so I could be wrong in my assumption that it needs to be flat. I may be - probably - am wrong about it.

Or maybe you have a punch that can be struck with a hammer. Or maybe the nibbler tool. Inquiring minds want to know!

As for me, I'm leaving the power cord through the strain-relief. I'm going to install a 3A fuse holder, but that's an easy-to-cut round hole. I'll just punch a dimple and drill it.

I am thinking I might like to mount a couple Centronics connectors though. I'm thinking about options for disk drive connections. I don't want to just run the 34-pin cable out through a hole. So I might use Mike's daughter board - the ones included with the FDC+ board - or I might use a 36-pin Centronics or some other connector. I just don't know yet.

I also thought about mounting a Centronics connector for a parallel port but I think I'm ruling that out. I'd rather just connect a printer to a serial port. Could have digital and analog I/O though. But now I'm getting way ahead of myself.

Too much rambling. Sorry. I'm just really curious how you plan to punch or cut the hole for your AC panel mount connector.
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Re: Altair 8800c build thread

Postby KenF » May 22nd, 2022, 7:45 am

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?


No. That is an Internet myth. Or would be if the Internet had been around back then. Many legitimate sources give that Intel did not sell or pass on "Cosmetically Defective ICs."

One source
https://en.wikipedia.org/wiki/Altair_8800

Intel made the Intellec-8 Microprocessor Development System that typically sold for a very profitable $10,000. It was functionally similar to the Altair 8800 but it was a commercial grade system with a wide selection of peripherals and development software.[24][25] Customers would ask Intel why their Intellec-8 was so expensive when that Altair was only $400. Some salesmen said that MITS was getting cosmetic rejects or otherwise inferior chips. In July 1975, Intel sent a letter to its sales force stating that the MITS Altair 8800 computer used standard Intel 8080 parts. The sales force should sell the Intellec system based on its merits and that no one should make derogatory comments about valued customers like MITS. The letter was reprinted in the August 1975 issue of MITS Computer Notes.[26] The "cosmetic defect" rumor has appeared in many accounts over the years although both MITS and Intel issued written denials in 1975.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 23rd, 2022, 5:23 pm

Yes, that's what Ed Roberts said in his interview too - at least the part about 'em going into Altairs. Interesting how those rumors still persist.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 25th, 2022, 11:00 am

I made a little more progress last night.

I setup a minimal system with CPU, front-panel, FDC+ and SIO. I have two of Eberhard's serial boards, so I thought I'd test them.

As an aside, I have two of pretty much everything - which really helps in troubleshooting - but so far, every board has worked first try. So I didn't really need to swap boards and compare. Until now.

Martin had run out of his "G" version boards because he was working towards a version "H." So he sold me his last version G and one of his first version H boards. That's kind of cool. The version G board has his Amon monitor ROM installed, and the version H has ROM BASIC on it.

The version G board worked right away. I setup Mike's FDC to disable ROM and to have RAM up to 7F00. Could have gone higher on RAM but I set it that way anyway 'cause the ROM BASIC on the H board starts at C000. So I set the memory to 32K, inserted the G SIO board, powered-up, started at address F800 and got the Amon prompt on the console. Too easy!

Then I put in the H board and started at C000. Nothing. Nada. Nobody home. But I did see some little blinky-lights. So I single-stepped through a few locations and it appears that the program is running. I'm using the same cables, so that's not it. But it could be that I have the board configured improperly, like for different signal levels (TTL versus RS-232) or maybe for 20mA current-loop. Not sure yet.

I wrote Martin, so when he has time, he might reply back with something I've overlooked, or with some "secret sauce" things to try. Or maybe I'll make some time to put a scope on the end of the interface, look at the signals there and work my way back. Might just continue assembly with the working parts I have so far. Maybe a little of all that.
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Re: Altair 8800c build thread

Postby AltairClone » May 25th, 2022, 10:44 pm

If you’re running Altair ROM BASIC, you have to set the front panel sense switches after examining C000h but before depressing RUN in order to tell BASIC the type of serial board up you’re using. See my notes here: https://deramp.com/downloads/altair/sof ... ReadMe.pdf

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Re: Altair 8800c build thread

Postby Wayne Parham » May 26th, 2022, 8:43 am

Oh very good, Mike, thanks so much for the heads-up! I'll try that later today and report back!
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Re: Altair 8800c build thread

Postby Wayne Parham » May 27th, 2022, 10:17 am

Sadly, no luck. I did as you described, and still nothing. But it was good that you pointed that out to me 'cause I hadn't seen that before. Definitely important "secret sauce" for running BASIC in ROM on the Altair.

As I was fooling around with it, I realized that - as long as I was hitting the right port - I would have expected to see something, even if baud, parity or stop bits were wrong. I expected to at least see garbage. So since I didn't, I put a scope on the output line. It was +12, which is weird 'cause it should have been -12 at idle. And no pulse train (of data) when execution begins at C000.

So after seeing nothing but the +12 on the line - which is the output of the 1488 driver chip U7, pin 1 - I then put the scope on pin 6 of U4, the 6850's TxD output pin. Nothing there. So for whatever reason, the system isn't pushing stuff into that UART.

I might fiddle around some more with switch settings and jumpers. Could be a switch is in-between contacts or something. If so, parts of the board might not be addressed properly. I think the EPROM is enabled and set right, 'cause it looks like the program is running. But it could be jumping into nowhere and just "look" like it's running something useful. Could be running in circles doing nonsense. Or the ACIA chips might not be addressed right. But it is odd that it sets the TxD line high at idle, I mean coming from the 1488.

As an aside, Martin added a bank of DIP switches for the sense input you were talking about. One switch enables the feature and others set the COM configuration. My board isn't populated with that switch - so all are open - but maybe after I get the board running I'll add the switch. Seems more natural than doing it through the front-panel.
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Re: Altair 8800c build thread

Postby Wayne Parham » May 28th, 2022, 9:03 am

I've stopped troubleshooting the 88-2SIOJP rev H board. If it weren't a brand-new board, or if I had assembled it myself, I'd probably go further. But I think now it makes more sense to let Martin have a go at it. I'll just use the rev G board for now and proceed.

So last night, I soldered up the little 50-pin to 34-pin daughterboard adapter for the diskette drive. I configured the FDC+ board for drive type 5 and connected everything up to a Teac drive. Entered Mike's BIN loader using the front-panel, started execution at location 0. Tried to upload PC2FLOP.COM a couple times but I didn't see front-panel activity during the send so I doubted whether it worked.

I decided to go another way, and started AMON, so I could use the HEX file uploader. Bada-bing, bada-boom. Uploaded first try. Started execution at 0100h and voila, "Houston, we have lift-off."

Told it to write to drive 0 and to transfer using port 0. Used Xmodem to send CPM22-48K-HDF.dsk and it started loading, and gave the progress indicator. My serial port is set to 9600/8/1/N so it wasn't incredibly speedy. But it was going!

I could see the diskette light go on, and eventually it kicked on the motor. I guessed that it was caching up data before actually writing it.

But then it stopped. The progress indicator was at 3.4%. Eventually, the Xmodem transfer decided to give up and removed the progress indicator message altogether.

Anyone see this kind of behavior?

Later this weekend, I'll try a different drive. Maybe I'll think of some other things to try too. Could try a different cable, one that just uses 2,3 and 7 - in case handshaking tripped it up. My cable has all the handshaking lines. Doubt that's it, but maybe. Any other suggestions?
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