Forum Rules and Etiquette

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This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

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Help! I'm running out of slots!

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    Help! I'm running out of slots!

    My NorthStar Horizon has 12 slots. A complete 256 color Microangelo system takes 9, 10 if the "genlock" (what is a "genlock"?) also takes a slot. I think it supports up to 8 CPU boards. Then there's the FPU, floppy controller, hard drive controller, tape backup controller, 6 or 8 RAM boards, a Cromemco 8PIO, a clock, Screensplitter video board, A/D board, sound board(s), cassette, video digitizer...

    Maybe I should quit while I'm only up to 38 slots.

    I don't want to replace the motherboard. I want to keep it original. Has anyone here ever connected a second motherboard, using heavily shielded and buffered cables, to an S-100 system with limited slots?

    Oh yeah, the interface board in each box would take another slot.

    I doubt you could get past 2-3 MHZ with that arrangement. May want to consider boards with multiple functions.


      Ever heard of a "slot box" for a pee sea? If they can do it, so can we.

      First, I will probably have to build the board myself. I would put buffers at each end of every signal. I would use the kind of ribbon cable with the shield on one side, like an Apple disk drive cable. Every other wire would be a ground. I've seen ribbon cables with shielding around them and insulation over that.

      It still might not work. But if I can add slots to my pee sea, Why can't I add them to my S-100 system?


        Hi! Bus length, termination, cross talk, reflections, signal propagation delays, voltage drop, floating grounds, etc. There are many reasons but John is correct. Large S-100 bus motherboards/chassis generally have problems with signal quality.

        The PC expansion slot took a <5 MHz bus with 5 or 8 slots and expanded it with 5 or 8 more. Total slot count is between 10 to 16. Typically slow IO boards went in the expansion chassis like serial or parallel ports not memory or video boards. That is a much more forgiving situation than trying to add another general purpose 12-24 S-100 slots to a NS Horizon S-100 bus.

        S-100 bus termination and careful design can limit the problems but not make them go away altogether. Many of the large format S-100 chassis had a lot of slots but often many slots were left empty for cooling, etc. Completely full large S-100 bus system (>12 slots) are pretty unusual and tricky to get working reliably. Adding an expansion chassis to that would be extremely difficult.

        I'm not saying it is impossible but nearly so -- I would love to see it happen. IMO, the practical limit is about a dozen slots (what's available in a NS Horizon). There are exceptions but not very common.

        Thanks and have a nice day!

        Andrew Lynch


          Originally posted by lynchaj View Post
          I'm not saying it is impossible but nearly so -- I would love to see it happen. IMO, the practical limit is about a dozen slots (what's available in a NS Horizon). There are exceptions but not very common.

          Thanks and have a nice day!

          Andrew Lynch
          Most Cromemco systems had 22 slots; a multi-user configuration could indeed fill most of those, especially if you used small memory cards and I/O cards with few ports.

          But I'd agree that an external bus extender would be a bit of a challenge to keep clean, considering that even some ordinary single small-count backplanes had problems with noise once they were heavily loaded (not to mention that you're going to need some serious power and cooling).

          But hey, more power to ya; Andrew, don't you have some backplanes to sell?


            22 S-100 slots, given the uneven quality of third-party boards would be impossible in a mixed-board system. It's conceivable that a single vendor's boards (i.e. Cromemco) might be made to work in such a system. But most vendors didn't pay too much attention to bus loading and capacitance that tends to many-card backplanes.

            You could bridge two backplanes using some transcievers and latches, synchronized to the bus, but it would cost you at least one wait state.
            Reach me: vcfblackhole _at_ protonmail dot com.


              Hi Mike! I designed a small actively terminated S-100 backplane PCB for benchtop repair/debugging of S-100 boards. Its not really intended for building a full blown S-100 system although you could use it if you wanted.

              The N8VEM S-100 backplane PCB supports 6 slots. I kept it small to fit on the benchtop and also to minimize noise issues. That being said, S-100 systems are notorious for noisy backplanes.

              Thanks and have a nice day!

              Andrew Lynch


                Never had any bus problems with the Cromemco systems, but the quality (and price) was a little higher than most of the other S-100 stuff, and yes, they were all exclusively populated with Cromemco cards; for some of the high-speed stuff they used a secondary bus across the opposite card edge using ribbon cable and headers.

                Just thought the OP might be interested in one of your boards (if there are any left); might be just the thing for this kind of experiment.


                  Ok, maybe I should limit it to one or two CPU's, and limit the Microangelo system to 16 colors. The Cromemco 8PIO is a permanent part of the system - it controls a "switch register" (wannabe front panel) installed in the front of the computer. I will probably remove the Screensplitter. So except for the 8PIO, all boards will be NorthStar or Microangelo. Did NS ever make a Z80 equivalent of their 8088 Single Slot Computer?

                  What about one of those backplanes like the Archive or SOL-20 used? It would fit in the back slot, but the cables would get a little crowded.