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Digiac 4500 Fixup/Repurpose

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    #16
    Gorgo did deliver on documentation for the Digiac 4500, and I've continued my efforts into rebuilding/repurposing this one. It's pretty much done now!

    I think we left off with "it seems to not be totally dead," w.r.t. the CPU board. I ended up customizing GWMON-80 for it since the Digiac monitor was a real pain to use. That worked fine, but the Digiac CPU board didn't really fit with what geneb wanted to do with the machine. We decided to go with a different board set but keep as much as the Digiac stuff as possible. It turns out, that's limited to...the floppy controller!



    This is the floppy controller. From disassembling the ROMs that came with the system, it was starting to look familiar. Turns out it's 100% compatible with the SD Systems VersaFloppy II, a very good card indeed. The first check for signs of life was to get John Monahan's VF2 Diagnostics running:



    Success! I was able to format, read, and write disks. I started out with 5.25" disks since that's what the machine came with, and eventually switched to 3.5" HD drives. The process of getting CP/M up and going was nontrivial. I started with Bruce Jones' VF2 code, which Herb Johnson makes available through his website. Due to issues with getting it going "bare metal" on the Digiac, I ended up restoring an old North Star Horizon that had been on the to-do list for some time. With CP/M 2.2 running on the North Star, it was *much* easier to get the VersaFloppy II code from Bruce Jones figured out. There were some errors in the code, some of which I'm pretty sure are bit-flips from its transfer to modern computers. There was also a lot of extraneous stuff in the Bruce Jones code. After a week or so of hacking on it, I was able to get CP/M 2.2 for the VersaFloppy II running directly on the North Star Horizon, in place of the North Star hard-sector disk controller. I will fully document that process separately -- I know a *bunch* of people want CP/M going on their VF2 cards.

    With CP/M going on the North Star, it was time to set up a more permanent system in the Digiac chassis. I ended up going with the following board set:

    * Industrial MicroSystems 4 MHz Z80 board, with power-on jump
    * Dahlgren 64K static RAM board (uses 6116s)
    * Digiac VF2-compatible floppy board
    * Seattle Computer Products 400C 4-port serial board (8251A based, with an interrupt controller)

    To get going, I customized GWMON-80 for the SCP 400C board and burned it to a 2716 EPROM, which I stuck in the Dahlgren SRAM board (part of the reason for choosing it). Eventually, I used one of my reproduction Solid State Music IO-2 boards to make up a board with 2K of EPROM (2716), ROM control circuitry so it can be switched out via software, and parallel I/O. It sure did seem like a shame to only use a bit or two of the IO-2's parallel I/O, so I talked to geneb and we decided I'd build a mini front panel for the machine!



    This is the aluminum blank for the mini front panel's inner panel. I used a two-piece system, kind of like an Altair front panel, where the inner panel is structural and the dress panel is just that, basically a legend that bolts on. The inner panel is made from 1/4" extruded aluminum plate, which was not flat enough for the purpose as it came from McMaster-Carr. I faced it off in the milling machine, and added a small step on the back side for mounting.



    The above image shows the blank mounted up. The step in the back lets the back of the panel sit in-plane with the back of the Digiac's sheet metal front. Mounting is via four #4-40 screws, with a flat washer to distribute force and a nylon washer to deform and hold it in place. So far, this is a very solid mounting solution.



    Here's the first somewhat decent dress panel. It's made from black anodized aluminum, 0.040" thick. I was hoping that the laser cutter at our makerspace could directly engrave the anodized aluminum, but the results were poor. I came up with a system by which I was able to produce a mask to etch the panel using sodium hydroxide (lye), which I will document separately.



    Closeup of the prototype dress panel. You can see some "ghosting" where this one was etched over an existing attempt to direct-engrave with the laser cutter.



    This is the final dress panel, not yet sheared out and punched. It uses the same etching technique as the prototype, somewhat refined. I'm using Hershey fonts here, which are vector single-stroke fonts more suitable to engraving. They give a much cleaner etch.



    Closeup of the final dress panel. I etched the centers for all of the holes to be punched and the shear lines, this is very helpful!



    Before I punched out the prototype dress panel, I drilled 1/16" holes through the centers and used it as a drill guide for the inner panel. Above is the finished inner panel. Holes for LEDs were drilled undersize and then reamed with a #2 taper pin reamer, which gives a press fit with 5mm LEDs. The recesses in the front are to allow mounting nuts for toggle switches to sit below the inner panel's surface.



    Closeup of the front of the inner panel.



    Back of the inner panel. Notice the bottom-right corner error, the Y-axis lock on the milling machine let go on me, I'm pretty sure there was a chip stuck between the wedge that locks the Y-axis and the ways. Oh well, no real harm done. Tapped it from the front due to the error.



    Mini front panel assembled with the prototype dress panel.



    Mini front panel installed and running in the Digiac system! I of course had to load BASIC and run a Cylon scanner program



    Here's the wiring behind the mini front panel, we decided that a PC board wasn't really necessary for a one-off.
    Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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      #17


      Today I finished assembling the system!



      Here's the card cage. From front to rear: reproduction SSM IO-2 for ROM and mini front panel, IMS 4 MHz Z80 CPU, Dahlgren 64K SRAM board, Digiac floppy controller, and SCP 400C serial board.



      Back of the machine, showing ports and labels.



      Closeup of the back panel ports. I like to use a DB25F for the console port and DB25M for other serial ports.



      Screenshot of the system booted up and running CP/M 2.2. With the DISK/MONITOR switch on the front panel in MONITOR mode, the system comes up to GWMON-80. The `b` command will boot the system from the first floppy drive.
      Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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        #18
        Squeeeeee! *ahem*



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

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          #19
          When did S100 boxes start using male DB-25s for comms? Most of the ones I'm familiar with used DB-25Fs (yes, I know that a computer should be DTE and not DCE, but that's how it was done).

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            #20
            If memory serves, that's the way I wanted it.

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

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              #21
              Originally posted by geneb View Post
              If memory serves, that's the way I wanted it.

              g.
              Yup, when the guy who's getting the box says he wants it that way! The cables from the SCP board to the backplate I/O connectors are not original, I didn't get any with the SCP board so I made up custom length cables (and yes, they'll even reach beyond the last slot!)

              Of the vintage systems I've worked on, I've seen a mix. Can't say when they were last hacked on, of course.
              Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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                #22
                While going through some random 5.25" diskettes from the previous owner of the equipment that the Digiac 4500 was in, I came across its system diskettes! They were labeled simply, "CP/M Master." Given the number of other CP/M boxes that he owned, that wasn't a very good label

                Going through the dump of the disk with IMDV suggests that the Digiac board set really is a (licensed or unlicensed) copy of the SD Systems SBC-200, VersaFloppy II, and EconoRAM. There's a SD Systems copyright notice still buried in there!
                Check out The Glitch Works | My Retro Projects | Vintage Computer Services | Glitch Works Tindie Store -- Vintage Computer Kits and More

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                  #23
                  Nice job Glitch bringing this old beast back alive.

                  Having built a few Orange II computers at one time I'm shocked to see that there was copying going on during the old days. <G> I standardized on female DB25's on my S-100's because that's what most of mine had originally and not having to use null adapters or different cables for each one connecting to terminals and printers seemed like a good idea.

                  Crazy old guy with a basement full of Pentium 1 laptops and parts

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