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Building an Altair 8800 like it's 1975!


tep392

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My next project is to reproduce the MITS 88-UIO board. It provides a serial interface for a terminal plus a cassette interface all in a single board.  The first MITS tape interface was a two board deal with a modem board and serial board to communicate with the computer. You would still need another serial board to communicate with a terminal device.  So this board will provide the function of three cards on a single board.  Other advantages are that port settings be configured with dip switches, where the original SIO board required soldering jumpers to reconfigure it. The problem is that MITS released this board just before they went out of business and it hadn't yet been mass produced. There are just a few boards out there, but one of them is in the hands of the guy that designed my reproduction case. He has sent me some hi res photo's that I am going to use to reproduce the board.  I also have schematics from the manual to help figure out the traces that are hidden under components.

 

IMG_4395.thumb.JPG.4da092552fc6b8b287a365cd5718fb97.JPG

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30 minutes ago, bent_pin said:

Will you need to build a new backplane for that or can you chain the backplanes?

I have one slot still open for the new card.  I will eventually chain another backplane on when I want to install more boards. Played a little "kill the bit" tonight. :) It's one of the small programs that built into the panel interface card.

 

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On 12/24/2023 at 2:29 AM, ClausB said:

Nice!

 

In 1976 in Mr. Dyk's class I learned BASIC on two 16K 8800a micros, one with a teletype and paper tape and cassette, the other with a video terminal and cassette. It was so fun I stayed after school often. We started a club and ran a computer dating service. The story here, page 8:

cn0777.pdf 2.87 MB · 1 download

Thanks for posting the article!  My first experience with computers and BASIC was in my high school lab of TRS-80 Model 1's. I would stay after school to play Star Trek and mess around in BASIC.  The next year they updated the lab with Apple IIe's.  My electronics teacher had a system in his office that I think was S-100 based, but I can't remember which system he had. 

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  • 2 weeks later...
On 12/12/2023 at 12:24 PM, tep392 said:

My next project is to reproduce the MITS 88-UIO board. It provides a serial interface for a terminal plus a cassette interface all in a single board.  The first MITS tape interface was a two board deal that also required a serial I/O board.  So this board will provide the functions all three cards with a single board.  Other advantages are that port settings be configured with dip switches, where the original SIO board required soldering jumpers to reconfigure it. The problem is that MITS released this board just before they went out of business and it hadn't yet been mass produced. There are just a few boards out there, but one of them is in the hands of the guy that designed my reproduction case. He has sent me some hi res photo's that I am going to use to reproduce the board.  I also have schematics from the manual to help figure out the traces that are hidden under components.

 

IMG_4395.thumb.JPG.4da092552fc6b8b287a365cd5718fb97.JPG

I'm making progress on this board.  All components are confirmed and I have orders placed.  The board design is coming along well. I've finished the bottom of the board, the easy part, and now need to work out all the top side traces that are hidden by components.

BoardProgress10Jan24.thumb.png.33f323dcb7d0697a56e9cf49b137b01c.png

 

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Front side view. I used the board to read in the Altair 8k BASIC cassette bitstream and then write it back out to cassette using the KC standard.  I had to write the copy program and key it in on the front panel. Some old school pirating there. :)

 

Proto.thumb.jpeg.4557fb0159544bb6aff0304daa0c5ed9.jpeg

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For those of you who are interested in the inner workings, here are some images from the adjustment of the PLL.  

 

This is a Kansas City standard test signal (1200/2400Hz) with the corresponding digital conversion. I adjusted for KC standard.

KC_Alignment.jpg.6593c123253b4e13d1aef4722125d126.jpg

 

After adjustment, switching to MITS standard test signal (1850/2400Hz), and checking digital signal is good. The adjustment of the PLL is not the same for both standards. There is a bodge added on backside of board to switch in a parallel resistor when using MITS standard so that the trim pot doesn't need to be readjusted between the two.

MITS_Alignment.jpg.1f7146ec63fa45b0fa91f9b9a6c8ea26.jpg

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