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

  1. #81
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    If you're up for fooling with your breadboard setup, here's something we could try. Here's the Tandy Faxback that mentions the 768k configuration for the 1000 HX when equipped with the 25-2062:

    ftp://ftp.oldskool.org/pub/tvdog/tan...back/01083.pdf

    I can't find a schematic or manual for that board, so the two possibilities are:

    1: That board is just 512k in a linear array, nothing special, and the software it comes with tweaks the Big Blue mapper to enable using the extra memory that's present but otherwise hidden in this configuration, or:

    2: That board has a mapper on it that sticks its extra RAM in a UMB or banks-switches it.

    If it's possibility #1, then in principle you might be able to run the RAMdisk driver that's compatible with the Tandy board with your breadboard setup. This looks like the right driver:

    ftp://ftp.oldskool.org/pub/tvdog/tan...s/mempdrvr.exe

    It's worth a shot?

  2. #82
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    ... still failing to find a detailed theory of operations for the 25-2062, but I did find this which makes me suspect that the RAMdisk driver isn't going to work with the "jumpered straight across" setup. (It's still probably worth a try just to make sure.)

    Screen Shot 2019-06-14 at 1.39.04 PM.jpg

    That it has different jumper settings for the EX and HX suggest that it could be mapping that 128k into the UMB areas, because EX and HXs have different areas that are open. (If the manuals are to believed the EX has everything from C000h to EFFFh open, while the E-page is occupied by DOS in an HX.) Wish someone had one of these cards to run Check-It on. If it is using UMBs it'd be neat to know *which* it uses.

    Anyway, *If* that's the case then I think we can give up on hopes of easily getting "free RAM" in the A-page or eliminating the video page steal from conventional, but that's fine, it's a Tandy 1000 and that's the cost of admission. The question is still then what's happening with the straight-across jumpered setup counting to 640k and working. The wording of the manuals do suggest it could be counting up, seeing that there's 512k of expansion memory, and based on that it's loading the BB register with the value set that says "Map at 8FFFFh, and Big Blue RAM == 128k", therefore disabling 128k of the built-in RAM. The other possibility is the BIOS is loading for the 384k/256k split and 128k is conflicting but it's not caring. Again, it would be really nice to be able to tell for sure which.

    The safest route if we can't figure out if contention is definitely *not* happening is to limit to 384k of the SRAM. If we're going for the ultimate in minimalism... I think you could do it with a single 74LS00? (Logic: one NAND gate across A18 and A17, that will give you a "1" for every condition but 11. Use a second to invert A19, that'll give you a 1 when you're in the bottom 512k. Then use a third gate to combine those two, that'll give you a "0" for enable when A19 is low *and* you're in the first three 128k pages of the SRAM chip.) That's a three-chip conventional backfill board *including* a '245 buffer.
    Last edited by Eudimorphodon; June 14th, 2019 at 02:45 PM.

  3. #83
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    You could always just add a 22V10 to be completely safe.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  4. #84
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    This is a surprise. I had no idea they had a 512K board!

    Quote Originally Posted by Eudimorphodon View Post
    The safest route if we can't figure out if contention is definitely *not* happening is to limit to 384k of the SRAM. If we're going for the ultimate in minimalism... I think you could do it with a single 74LS00? (Logic: one NAND gate across A18 and A17, that will give you a "1" for every condition but 11. Use a second to invert A19, that'll give you a 1 when you're in the bottom 512k. Then use a third gate to combine those two, that'll give you a "0" for enable when A19 is low *and* you're in the first three 128k pages of the SRAM chip.) That's a three-chip conventional backfill board *including* a '245 buffer.
    That would be my ideal setup anyways. No frills, straight "plug and play" memory expansion that people wouldn't have to worry about. Most people won't have any idea of what to do with UMB anyways.
    I don't have a '00 on hand (add a couple of those to my next Digi-Key order to have on hand), but I'll see if I can kludge something else together.
    My vintage systems: Tandy 1000 HX, Tandy 1000 RSX, and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, usually takes less space, and is less likely to lead to a fatal accident.

  5. #85
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    That idea of using NANDs was the first thing I could think of to do decoding with one chip (not counting programmable logic). A '138 or '139 plus an '08 or similar is the easy two chip solution.

    Did you try running the RAMdisk driver on the jury-rigged system just for laughs? Now that I was finally able to dig up the fact the original 512k board had those jumpers on it I'm not optimistic, alas, but it still might be worth a shot just to make sure.

    I'm still waiting for all my from-China goodies but I did get the extra-tall Raspberry Pi stacking headers yesterday. There's good and bad news on that front:

    Good: a set of them stacked together has essentially identical spacing to the Plus slots. If I stick a set on my modem card it rests perfectly in the middle slot position when the stack is inserted into the EX.

    Bad:

    Apparently I misunderstood, at least on the headers I bought the "xtra tall" spacer plastic can't be moved. As a result any board using this idea is going to rest *really* close to the bottom of a real Plus card stacked on top.

    Also, I apparently completely wiffed on the fact that these 2xXX headers aren't designed to fit snugly together, so in addition to having to cut one header down from 2x20 to 2x11 I had to file most of the wall off the cut end to make it fit the pin spacing. It works but, man, I wish 62 pins were a standard size.

    Also, possibly important data point: maybe the modem card is an outlier, but from actually extracting it I can say that you might need to leave a "gutter" almost one inch deep on the component side to be absolutely sure of no interference. See how this transformer essentially rules out fitting this card on stacked headers at all.

    20190615_123157.jpg

    And another shot of the bottom of the modem card.

    20190615_123316.jpg

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