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

  1. #521
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    The SX was the first 1000 family machine to have the DMA controller integrated onto the motherboard. I'm guessing Tandy concluded that since they didn't need any "extra" lines to implement the DMA control bus anymore they might as well eliminate their proprietary mapping of those pins.

    Thanks to some help in the other thread I was able to solve the particular issue that was vexing me, IE, not actually being able to use IRQ2 for the Realtek 8019AS ethernet card despite the hardware supporting the mapping. As discussed on that thread IRQ2 is itself kind of an interesting edge case for usability; it's fine with XT-vintage hardware and software, but because the AT architecture pirates IRQ2 and attaches the same pin to IRQ9 on the second interrupt controller apparently some software that otherwise runs on an XT, like the "native" packet driver for the Realtek card, won't run with the card on IRQ2 because it makes AT-centric assumptions. Technically it's a stupid bug in the driver that could probably be easily fixed, but I wonder if there are other examples of that sort of thing lurking out there just waiting to bite.

    (The workaround here was leveraging the card's NE2000 compatibility and using an 8-bit compatible NE2000 driver that allows manually specifying the I/O and IRQ settings accurately.)

    Another thing I've started noticing is there really does seem to be a non-trivial amount of DOS software from the early 90's and later that doesn't like 8088 CPUs but will run on V-20s. So far I've left the 8088 CPU in my HX test rig, and I can confirm that in addition to the current version of Cutemouse being a problem there seems to be an issue with DOS Kermit; a weird, funky issue that's going to force me to break out my second eBay V-20 to absolutely confirm it. Fun stuff!
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  2. #522
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    The great thing about the Tandy 1000 line, unlike the early IBM 515X series, is that pretty much everything you needed to function was already on board, and didn't cost you an arm and a leg.

    I contemplated, briefly, whether or not it would be possible to gang another 8259 to add more IRQs to the EX/HX. I think it IS possible, but it would require either cutting traces, or socketing a special daughterboard, making a custom psudo "16"-bit expansion bus for the extra IRQs, etc.... Not really worth it, IMO, but could be fun as a thought experiment.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, 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.

  3. #523

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    Quote Originally Posted by Eudimorphodon View Post
    Well, I've been having some fun with in-depth testing. So far I think my hardware is actually just fine, but it's kind of amazing some of the machine-specific quirks I've run into.

    Does anyone who's keeping an eye on this thread happen to use an Ethernet card in their EX/HX using whatever kind of adapter you've made, bought, or otherwise magic-ed into existence? I'd be curious to hear what you're using and what hardware resources you've assigned it.
    I'm using a WiModem 232 for all external comms - works great for my needs.
    My Retro Collection:
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  4. #524
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    Quote Originally Posted by Eudimorphodon View Post
    The SX was the first 1000 family machine to have the DMA controller integrated onto the motherboard. I'm guessing Tandy concluded that since they didn't need any "extra" lines to implement the DMA control bus anymore they might as well eliminate their proprietary mapping of those pins.
    They still had pin B8 (slot-8/card select) as an "audio in." I'm guessing that was for the modem card?
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, 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. #525
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    Quote Originally Posted by blackepyon View Post
    The great thing about the Tandy 1000 line, unlike the early IBM 515X series, is that pretty much everything you needed to function was already on board, and didn't cost you an arm and a leg.
    Oh, don't get me wrong here, I'm in no way ragging on the Tandy 1000. The... eccentricities, I'm running into are all part of the fun. I'm just getting amused at how the well of differences between these machines and regular old boring PCs doesn't seem to be showing any sign of bottoming out.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  6. #526
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    Quote Originally Posted by blackepyon View Post
    They still had pin B8 (slot-8/card select) as an "audio in." I'm guessing that was for the modem card?
    Have you checked the schematic yet to see where that goes? Does it just go to the audio amplifier circuitry (via a crude mixer?) to act as a remote speaker for a card, or is it more sophisticated than that.

    Ironically the EX modem has a big 'ol speaker on it, so I'm kind of doubting it used it.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  7. #527
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    Quote Originally Posted by Eudimorphodon View Post
    Have you checked the schematic yet to see where that goes? Does it just go to the audio amplifier circuitry (via a crude mixer?) to act as a remote speaker for a card, or is it more sophisticated than that.
    That seems to be what it is. B8 goes to U25, a "14529." Google thinks it's a Ford window switch, so can't say exactly, but it's directly in front of the op-amp, and the 3-voice sound chip also feeds into it, so it's some kind of sound controller. Grab the technical reference manual from Oldskool if you don't already have it.

    The EX/HX have B8 not connected to anything, so you would't get the pass-through audio there.
    Last edited by blackepyon; Yesterday at 11:27 PM.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, 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.

  8. #528
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    Quote Originally Posted by Eudimorphodon View Post
    The... eccentricities, I'm running into are all part of the fun. I'm just getting amused at how the well of differences between these machines and regular old boring PCs doesn't seem to be showing any sign of bottoming out.
    Likewise.
    I find it interesting how Tandy managed to combine so many things to save space and costs. Such as mentioned above, how the EX/HX have the V-sync circuit coming out of the interrupt controller. The keyboard controller has pins controlling sound and floppy, and the parallel port controller manages some of the floppy controls as well. And the relationships change between each model.
    My vintage systems: Tandy 1000 HX, Tandy 1100FD, 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.

  9. #529
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    Quote Originally Posted by blackepyon View Post
    The keyboard controller has pins controlling sound and floppy, and the parallel port controller manages some of the floppy controls as well. And the relationships change between each model.
    While working on my HX project I noticed all the weirdness hanging off the keyboard controller. Of course, to make it more fun the EX/HX technically have two chips you could call the "Keyboard Controller" onboard, the actual "keyboard controller" in the standard PC sense (but also moderates those other peripherals) and the 8048 MCU that actually *scans* the matrix keyboard, which is only connected to the other by a few lines. I haven't verified by looking at the other machines' tech manuals but I assume that everything downstream of the serial connection between the 8048 and the keyboard controller is roughly what you'd find in a normal detachable Tandy 1000 keyboard. But it's still interesting, or maybe it's actually *less* interesting because it's too conventional, that Tandy laid things out like that. I'm kind of surprised they didn't come up with a unified keyboard controller ASIC that directly scanned the matrix.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

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