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Digiac 4500 Fixup/Repurpose

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    Digiac 4500 Fixup/Repurpose

    I acquired a pair of Digiac 4500s in a recent equipment pick-up. Digiac made a bunch of educational/training hardware and this is another product in the same vein. The main bit of the 4500 is a 22-slot sized S-100 chassis with a 12-slot backplane and drive bays in the front. There's one 5.25" half height floppy, a blanking plate for another, and a switch matrix. The switch matrix is used to induce faults in the system. I suppose the idea is that instructors set the faults and students have to figure it out, using the large remote front panel you can attach to the thing.

    The sad part of the story is, the previous owner died, and the person responsible for the estate threw out all of the documentation (I'm told it was complete, with classroom sets). There was a disk jammed in the floppy drive, but it turned out to be a moldy single-sided, double-density MS-DOS formatted floppy unrelated to the system. I have cause to get it up and going with CP/M as a friend wants a S-100 chassis for a particular project, and this is the one on hand that best suits his requirements (size, I/O, slots, integral drive bays, et c.). So, I started with disassembling the ROMs in the CPU board.

    Boy, what a trip! This thing is set up to have separate student and instructor modes, from what I can tell, and has a third option for talking to a mystery I/O device that I don't have. I suspect student mode was intended for use with a printing terminal -- it makes great effort in processing backspaces to preserve what might already be there. There are checks and traps around most of the monitor code that prevents the "student" sections from getting into the "instructor" sections, without physically disassembling the chassis and adjusting some internal DIP switches on the CPU card. But, today I got far enough into my disassembly that I was fully into reverse-engineering the command table and the subroutines for each command function. I found that B appeared to boot the disk controller in the machine.

    After digging far enough into the floppy boot code, it started looking kind of familiar. Turns out the Digiac controller is a modified/improved/cloned/compatible...or whatever...SD Systems VersaFloppy II. Of course, our own John Monahan has written an extensive test facility for the VF2. Since this system is Z80 based, I decided to give it a go. After patching up the console I/O for the Digiac "instructor" console (it runs at 9600 BPS, vs 1200 BPS), I organized it into a "load format" file -- you paste it into the monitor and it loads using the monitor's C command (Change memory, I suppose). The monitor is very clunky, everything works off of 0x0000 -- from what I've reverse engineered there's no way to set a starting address, either to change or to display!

    Anyhow, I loaded it, jumped in, and it worked perfectly! I had confirmed the controller was somewhat operative by putting a single-sided, single-density Cromemco CDOS disk in and hitting B -- it locked the machine up, but I could see the first sector of Track 0 in memory, in the sector buffer. But, with John's VF.COM program loaded, I could not only do double density, but I could also read double-sided drives (controller has a FD1795). So, that's all excellent news!

    The next step is of course to start building a CP/M for the system.

    Here are some pictures of my setup:

    Full Imgur album: http://imgur.com/a/sdo3E

    Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

    #2
    Is the monitor spread across the three EPROMs on the board, or do they hold other software?

    g.
    Proud owner of 80-0007
    http://www.f15sim.com - The only one of its kind.

    Comment


      #3
      Originally posted by geneb View Post
      Is the monitor spread across the three EPROMs on the board, or do they hold other software?
      It's spread across all three ROMs (3x 2708 EPROMs, so 3K of monitor). The monitor includes at least boot code, I haven't fully finished reverse engineering it.
      Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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        #4
        Interesting. I've not seen code spread across multiple parts like that before.

        g.
        Proud owner of 80-0007
        http://www.f15sim.com - The only one of its kind.

        Comment


          #5
          Curious system. Even curiouser name--sounds like a formula for infants.

          Comment


            #6
            Hi All;

            The name made me curious about some boards that I had, only they were called Educomp, they are not S-100..

            But, they were for educating Students about the Principles of Computers..

            I have some of the Boards, but not all of them for a complete system, and of course no Documentation..

            THANK YOU Marty

            Comment


              #7
              The plot thickens! It seems their DRAM board is a copy/modification/whatever of one of the 64K SD Systems EconoRAM boards. I wonder if the CPU card is based off of a SD Systems SBC-100 or SBC-200 -- they do have a switch port at 0x7F, which I believe was used as an output port to control some onboard functions on the SBC-100/200.

              I suppose at least they copied a decent set of boards!

              I'll have to put together an image album of the whole system, once I get one fully cleaned up. I've got the large remote demonstration front panels, it's not yet clear whether they are just a peripheral device or a real hardware front panel. Unfortunately the front panels became homes to a number of mice -- when I picked them up, the guy helping me said, "huh, these are leaking oil" -- it was *not* oil.
              Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

              Comment


                #8
                I did a little digging yesterday and it appears that Digiac has been producing computer & logic trainers for quite a while. I also found a notice in a 1980 issue of Byte about them starting an MP/M user group.

                If you search eBay for "Digiac", you'll get a number of hits for logic trainers. No S-100 stuff though.

                g.
                Proud owner of 80-0007
                http://www.f15sim.com - The only one of its kind.

                Comment


                  #9
                  Yeah, they made a bunch of different educational stuff, from logic trainers through automotive ECU simulators, all seem to be aimed at the educational market. I'm sure they chose to use a S-100 box simply because S-100 was common and available at the time.
                  Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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                    #10
                    I'm continuing to dig to see what I can find. Can you look at some of the date codes on the CPU board's chips to give me a ballpark of when the board was made? I've got a full set of Kilobaud that may have ads for Digiac, but I'd like to narrow the search if I can. No Ctrl-F with dead tree copies.

                    Interface Age & S-100 Journal would be good places to look as well I think. There's not may of those online though.

                    g.
                    Proud owner of 80-0007
                    http://www.f15sim.com - The only one of its kind.

                    Comment


                      #11
                      Marty:
                      Exactly what documentation are you looking for? I have some that came with my 4500 and a boot disk or two. I can supply a few photos of my system now that it works, if you would like.

                      Comment


                        #12
                        All:
                        I have a working Digiac 4500 with CP/M boot disks. I also have some documentation, primarily student material. The 4501 display panel was used by students to enter programs and single-step through the code. We used these machines when I taught computer technology at a vocational school in Anaheim, CA back in the late 80s. If anyone needs info, I can hopefully supply it.

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                          #13
                          Wow, excellent! I'd love copies of any information you have! I've got two of these, one is being fixed up for geneb as described, but I'd love to get the other operational as it was intended!
                          Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

                          Comment


                            #14
                            glitch:

                            I'll scrounge together what I have and let you know when it's ready.

                            Comment


                              #15
                              Originally posted by glitch View Post
                              Wow, excellent! I'd love copies of any information you have! I've got two of these, one is being fixed up for geneb as described, but I'd love to get the other operational as it was intended!
                              glitch:

                              I have the first installment of my Digiac docs scanned. Give me an email addy to send it to. Please let me know if this is the kind of thing you're looking for. It's in PDF format.

                              gorgo

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