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I wish to create a new DMA/RAM expansion card for the Tandy 1000 line.

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  • rkrenicki
    replied
    Second revision PCBs arrived today, and I can get it to properly recognize all 640k with no errors.

    I do seem to have an issue with booting from floppy disks while this card is inserted, so I may have made an error in the DMA portion somewhere. There is an "extra" pullup resistor pin on the resistor bridge, so I am thinking that I might have missed one somewhere.. but I do not see it on the scans at all. The system boots from my XT-IDE just fine, and the floppy disk works fine with a Lotech 1MB RAM card installed, so I know the disk and drive are okay. It gets about halfway through the boot process and hangs if this card is inserted.

    Unfortunately, without a keyboard for my 1000A, I cannot do much diagnostics. My Keyboard Adapter project is also awaiting PCBs, so I am stuck in the water until they show up.

    Leave a comment:


  • Eudimorphodon
    replied
    Originally posted by rkrenicki View Post
    It also was used to speed up floppy I/O, and apparently some floppy image programs intended to write non-native formats rely on DMA to access the disk as well.
    ImageDisk does indeed tell you to drop dead. (I don't have a Tandy *with* DMA to compare, but without testfdc fails with the claim of "no floppy interrupt".)

    Leave a comment:


  • rkrenicki
    replied
    Originally posted by bladamson View Post
    I think the DMA controller on the older Tandies was just a convenient way to do dram refresh since the video circuit couldn't do it on the expansion ram. The fact that is enabled DMA with expansion cards was just kind of an extra feature. I don't think it's really needed on that class of machine. I mean, the main reason to have it would be to make an 8-bit SoundBlaster work, but I don't think anything that will run on that class of machine will support more than AdLib anyway for the most part.
    It also was used to speed up floppy I/O, and apparently some floppy image programs intended to write non-native formats rely on DMA to access the disk as well.

    Leave a comment:


  • rkrenicki
    replied
    Originally posted by Eudimorphodon View Post
    Tandy's board uses a PAL as part of the RAM control logic as well, it looks like the one you're working with is all generic logic.
    Yes, this board is the only one that I know of that does the job with off-the-shelf logic chips instead of a PAL, which is why I wanted to duplicate it. Once I am satisfied that I have duplicated it correctly, then I will make a schematic out of it and eventually add the DMA portion back into my EX/HX 3-in-1 design, continuing to use the SRAM as the memory chip.

    Leave a comment:


  • Eudimorphodon
    replied
    Originally posted by RetroGaming Roundup View Post
    Watching an HX running Klax in VGA with soundblaster support is worth the effort.
    Huh. Do you have some kind of accelerator card in your HX? I've tried the game on mine and it's borderline too sluggish to be playable even with the Tandy graphics.

    Leave a comment:


  • RetroGaming Roundup
    replied
    Originally posted by bladamson View Post
    I think the DMA controller on the older Tandies was just a convenient way to do dram refresh since the video circuit couldn't do it on the expansion ram. The fact that is enabled DMA with expansion cards was just kind of an extra feature. I don't think it's really needed on that class of machine. I mean, the main reason to have it would be to make an 8-bit SoundBlaster work, but I don't think anything that will run on that class of machine will support more than AdLib anyway for the most part.
    I hear you, and in a practical sense your right, but the ones that work are just spectacular when they do. Watching an HX running Klax in VGA with soundblaster support is worth the effort.

    Leave a comment:


  • Eudimorphodon
    replied
    Originally posted by bladamson View Post
    I think the DMA controller on the older Tandies was just a convenient way to do dram refresh since the video circuit couldn't do it on the expansion ram. The fact that is enabled DMA with expansion cards was just kind of an extra feature. I don't think it's really needed on that class of machine.
    Yeah, that's why I'm not particularly fussed about ever dealing with it. The one thing that having DMA gave you back in the day besides RAM refresh was most XT hard disk controllers rely on it, but given that most MFM hard drives and hard cards have subsequently turned into pumpkins there's not a whole lot else that really meaningfully works on an 8088-powered Tandy 1000 that DMA enables. When this thread was started I think the emphasis was on the idea that having it somehow improved the speed of the machine but it's in fact demonstrable that getting rid of it buys you a few percent better overall performance because the DMA controller isn't butting in every 15 microseconds.

    Not to discourage anyone, though! If there's a grand plan to do a digitized sound demo ala 8088 Domination that relies on a Soundblaster card in a Tandy 1000 I can't wait to see it.

    Leave a comment:


  • bladamson
    replied
    I think the DMA controller on the older Tandies was just a convenient way to do dram refresh since the video circuit couldn't do it on the expansion ram. The fact that is enabled DMA with expansion cards was just kind of an extra feature. I don't think it's really needed on that class of machine. I mean, the main reason to have it would be to make an 8-bit SoundBlaster work, but I don't think anything that will run on that class of machine will support more than AdLib anyway for the most part.

    Leave a comment:


  • Eudimorphodon
    replied
    Fun stuff. I assume the intention in a later version is to ditch the DRAM multiplexing hardware and use a SRAM chip in place of all those 41256s?

    I did spend some time a while back puzzling over how the DMA section of the schematic of Sergey's XT matches up with what's implemented on the Tandy 1000's motherboard verses what was left hanging to be implemented in the card, and the impression I got was it wouldn't take *much* more than the 8237 and the 74573 and 74670 latches and a decoder for the I/O addresses if DMA was really a thing you wanted on its own. What's kind of interesting is I *did* find the schematic for Tandy's original board in the back of a PDF of 1000 technical information and its part count looks substantially higher; Tandy used a bunch of '244 buffers in front of the bus connector and otherwise seems like they were a bit more cautious. (Here's the PDF I found the schematic in. Maybe it might help you with troubleshooting if you don't have it.) Tandy's board uses a PAL as part of the RAM control logic as well, it looks like the one you're working with is all generic logic.

    Leave a comment:


  • rkrenicki
    replied
    I made a couple of bonehead errors, and there were a couple of sneaky traces that I missed on the first pass. I bodged on some fixes, and the system detects the memory correctly now, but I get some memory errors on the second bank. I think either the bodge wires or a possibly bad RAM chip are to blame for the errors.. but progress has been made.

    I fixed the design in sprint and sent them off for manufacturing again. I received an email this morning that they have shipped, so I should have an update by the end of the week.

    Leave a comment:


  • Eudimorphodon
    replied
    Originally posted by rkrenicki View Post
    The first draft boards have arrived. My 1000A and the logic chips needed to build this board will arrive tomorrow, so I will have initial testing done sometime tomorrow afternoon.
    [ATTACH=CONFIG]60536[/ATTACH]
    So how'd it work?

    Learning how to lay out PCB edge connectors is a thing I need to put on my list for my next project.

    Leave a comment:


  • rkrenicki
    replied
    The first draft boards have arrived. My 1000A and the logic chips needed to build this board will arrive tomorrow, so I will have initial testing done sometime tomorrow afternoon.
    20200428_202028233_iOS.jpg

    Leave a comment:


  • rkrenicki
    replied
    Screen Shot 2020-04-21 at 9.25.59 PM.png

    My first pass is complete. I sent it off to be fabricated for my initial tests.

    Leave a comment:


  • rkrenicki
    replied
    So, back to the original point of this thread... I was able to borrow one of the ATD T512CLK-A1 boards, and I stripped it and took high resolution scans of the PCB. I am going to first, make a duplicate PCB that is 1:1 of the existing design, likely in Sprint.. and second, map out the schematic in KiCAD for an updated version.

    I should have some progress quite soon.

    Leave a comment:


  • dJOS
    replied
    Originally posted by rkrenicki View Post
    The connectors arrived yesterday, and while I have not inspected every single one.. they seem to be perfect. They have a snug fit right out of the package, which I am very pleased with.
    [ATTACH=CONFIG]57684[/ATTACH]
    [ATTACH=CONFIG]57685[/ATTACH]

    The company I used is called Shenzhen RealRun Electronic Co. They typically have a MOQ of 1000, so I did end up paying a bit more per piece than normal for "only" ordering 250. They have a contact link on their Alibaba page here: https://realrun.en.alibaba.com/?spm=...621118397jvQ2r

    The part number is: "FH2.54*8.5-2*31P 0.64*0.4 Gold 1U PA6T"

    Otherwise, if anyone wants some small quantities, feel free to PM me here or since some of you know me on facebook, you can contact me there.
    Btw, Rob sent me a few to test and they are indeed excellent - I'll be switching to these when my current stocks of 62 connectors run out.

    Leave a comment:

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