Building one of Mike's Altair 8800C's - this is my questions thread!

[alank2]

I finally decided to give this a try and so far I've got the 9-slot backplane from Todd, an 88-2SIOJP pcb from Martin, and a 8080 Altair CPU pcb from Gary. The case, front panel boards, switches, and FDC+ controller are going to hopefully come tomorrow from Mike.

I will undoubtedly have a ton of questions as I build this thing so I'm starting this thread for them!

My first question is on the 9 slot back plane, does anyone know what type of connector it is for P2 in the corner? Is it a Molex 156 type connector? I can somewhat make it out in Mike's video.

 

[alank2]

2nd question - the Altair CPU board has different logic families on it 7404, 74123, 7406, 74L00, 74L74, 74L02 - is there any reason I can use the LS part for these?

 

[deramp5113]

2nd question - the Altair CPU board has different logic families on it 7404, 74123, 7406, 74L00, 74L74, 74L02 - is there any reason I can use the LS part for these?

For the most part, you should be fine with LS parts. In the oscillator circuit, you may have to adjust a resistor or two if you use an LS123, but even if you stick with a 74123, you’ll probably still end up needing to adjust from the published resistor values anyway. The ‘04 in the oscillator may need to be a 7404, but it will probably work fine with an LS04.

As far as the L parts go, I don’t recall one case in which MITS used a 74L part that its power or timing characteristics were actually required. In fact, there are several places MITS used a 74L for which it’s longer propagation delay pushed reliable timing to the hairy edge. This includes a couple of places on the CPU board. So by all means, eliminate the 74L parts. The generally accepts story is that MITS got a great deal on surplus 74L parts which were rapidly falling out of market favor and put them anywhere they could get away with one.

Mike

 

[deramp5113]

My first question is on the 9 slot back plane, does anyone know what type of connector it is for P2 in the corner? Is it a Molex 156 type connector? I can somewhat make it out in Mike's video.

I used the same connector as spec’d for J2 in the front panel board parts list here: https://deramp.com/downloads/altair/hardware/altair_8800c/Front Panel Manual.pdf

Mike

 

[DutchMaker]

I coincidentally purchased the Altair 8800 Clone (based on PIC24FJ128) this week from previous owner and will receive it today.
Will be keeping an eye out on this thread to see if I pick up something interesting.

Good luck on your build!

 

[alank2]

I hope you enjoy it DutchMaker!

 

[alank2]

What I've got so far!

 

[alank2]

For parts 1488 and 1489 for the 88-2SIOJP board, could MC1488N and MC1489N work?

Also, any substitute for 8212? I'm trying to use all LS parts in it.

 

[gekaufman]

For parts 1488 and 1489 for the 88-2SIOJP board, could MC1488N and MC1489N work?

Also, any substitute for 8212? I'm trying to use all LS parts in it.

74LS412 is a substitute for the 8212, but not easy to locate. 8212's do work fine.

MC1488N and MC1489N are perfect

 

[alank2]

Thank you!

 

[alank2]

More parts are in so a few more pics.

Edit, and yes, 22LR in boxes are great for placeholders that are heavy enough they won't move around.

 

[whartung]

Just marveling the idea that you actually have .22LR ammunition...

 

[gekaufman]

More parts are in so a few more pics.

Edit, and yes, 22LR in boxes are great for placeholders that are heavy enough they won't move around.

But they make really bad fuses...

 

[alank2]

That is true Gary! I finally soldered the backplane you sent me last night. Over 1000 solder joints! It is mounted.

The power supplies are in their positions, but not mounted yet. The thought here is that I can get at the terminal screws without having to unmount them from the chassis. I need to make up a quick CAD for the mounting screws to print out a drill template for them. Later today Mouser will bring a power cord and ring terminals so maybe I can get the power wiring done later on.

 

[alank2]

Much has happened in 4 evenings of messing around with this. Power supplies are mounted, high voltage and low voltage wiring is done.

Front panel interface board is done.

CPU interface board is done.

I had some considerable issues getting things working because (A) I substituted 74LS parts for the 74 parts on the clock, and (B) a few of the IC's I received were bad. Mike helped me greatly to troubleshoot things. I was able to change the resistors to get the 74LS parts to work for the oscillator and get it back to the proper timing for the 8080 requirements. We also figured out which IC's were bad and replaced them. One IC was even mis-marked!

I also learned that the TL866 EEPROM programmer can also test most logic parts! What can't that little thing do!

 

[alank2]

Here are some more pictures. I finally got Martin's 2SIOJP working and have the AMON monitor up and running on my DEC vt420. Turns out that a custom IC to fix an issue marked M was really a W. I hope to run CP/M on it later today with Mike's serial server on my PC connected to the FDC+.

All the IC"s have been replaced with TI chips (not seen in the pictures yet).