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Thread: NuXT - MicroATX Turbo XT Motherboard - 9.55MHz, UMBs, XT-CF, SVGA, HD Floppy, Serial

  1. #11

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    Quote Originally Posted by ejs View Post
    nztdm: FYI, that the Ad Lib clone card you're selling is based on my open source clone design which is licensed under CC BY 4.0 (Creative Commons Attribution 4.0 International). According to the terms of the license you need to give appropriate credit, provide a link to the license, and indicate what changes you made, if any. I'd appreciate it if you followed those terms by adding that information to your website and your eBay auctions. Thanks!
    No problem!
    I swear I did this, but looks like I had forgotten. I even had the README.md ready with the license and changes. I have updated my site listing (eBay listing will be able to be updated in a couple days).
    The changes I made are:
    > add footprint for Alps RK09K potentiometer. It's a 10th of the price of the large blue ones.
    > add part values, for helping kit-builders

    You have great attention to detail, right down to the shape of the pads

  2. #12

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    Quote Originally Posted by glitch View Post
    Excellent! If it's based on Sergey's work, you'll be releasing the design files in accordance with the GNU GPL v3, correct?
    Source!
    https://github.com/monotech/NuXT

  3. #13
    Join Date
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    What an amazing motherboard wish i have the money now to order one .

  4. #14

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    Hello how is the video card compatibility of this great motherboard? i would like to check several monochrome cards and monitors.

  5. #15

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    Quote Originally Posted by mixerjdp View Post
    Hello how is the video card compatibility of this great motherboard? i would like to check several monochrome cards and monitors.
    I have an IBM MDA, IBM CGA, Hercules, Trident VGA. All work fine.

    However I'm working out an issue. Currently, if you disable VGA, you can only use another VGA.
    Only boards without the VGA parts onboard will work with other video cards.
    The next version will likely have this fixed, or not include VGA.

  6. #16

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    Quote Originally Posted by nztdm View Post
    I have an IBM MDA, IBM CGA, Hercules, Trident VGA. All work fine.

    However I'm working out an issue. Currently, if you disable VGA, you can only use another VGA.
    Only boards without the VGA parts onboard will work with other video cards.
    The next version will likely have this fixed, or not include VGA.
    The VGA chip buffers all the bus lines and has lines to ram etc, pullups, pull downs etc. I dont see anywhere you detach all the pins on all these devices.

    I bet the monolithic VGA IC has reverse protection diodes and its activating it, making it 'sit on the bus' and mask its IO address etc.

    Dave has an interesting video on reverse protection diodes across IO pins powering an IC that has no power/gnd connected.

    https://www.youtube.com/watch?v=2yFh7Vv0Paw

  7. #17

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    Quote Originally Posted by BloodyCactus View Post
    The VGA chip buffers all the bus lines and has lines to ram etc, pullups, pull downs etc. I dont see anywhere you detach all the pins on all these devices.

    I bet the monolithic VGA IC has reverse protection diodes and its activating it, making it 'sit on the bus' and mask its IO address etc.

    Dave has an interesting video on reverse protection diodes across IO pins powering an IC that has no power/gnd connected.

    https://www.youtube.com/watch?v=2yFh7Vv0Paw
    Thanks for that. That was very interesting!

    I have since solved the VGA disable function. There is a pin in the datasheet that can disable it. It just wasn't clear. Will update the github with fixed schematic soon.

    Just one more issue to fix, which will hopefully be solved in the "rev 1.1":
    The serial port is unreliable. And a few factors can affect this for good or bad. It's a mystery to me at present.

    Testing method:
    Switch to 9.55MHz
    Open Windows 3.0
    See if mouse works and doesn't drop out after a few seconds
    Run checkit test with loopback connector
    Run CTMOUSE and then Lemmings

    Order of unreliability, from least reliable to most reliable:
    Windows 3.0 mouse (ultimate test it seems)
    Lemmings
    Checkit loopback test
    Text-mode mouse (works almost always)

    Seems like perhaps the VGA going into graphics mode makes it worse?

    Things tried:
    NI 16550 instead of TI 16C550
    Crystal instead of DIP oscillator
    Standalone serial card with same schematic
    No VGA onboard, but same VGA as standalone card
    Test voltages

    Things not tried yet:
    75C185 instead of GD75232 transceiver
    74ACT bus logic instead of 74F

    I wouldn't think trace lengths, ground return path length, noise, separate ground planes, etc would be too important at these low speeds, and as such i haven't paid much attention to these factors. I could be wrong.

    Notable things:
    >Installing another serial card as COM2, makes the onboard COM1 work perfectly (?? bus capacitance/termination)
    With the above, the COM2 serial card will now have the unreliability issue
    >Running at 4.77MHz improves things sometimes
    >With loopback test in checkit, when a port is unreliable, sometimes the registers test will fail, and sometimes the main test will fail at higher baud rates.
    >When creating an almost-NuXT out of Sergey's cards in a backplane, I can recreate the unreliability by moving the serial card to the last two ISA slots, furthest from the CPU SBC card.
    The serial circuitry -is- on the other side of the PCB as the bus/CPU circuitry on the NuXT. Maybe laying the board out again with the serial circuitry close to the bus will solve it. That would be nice, but wouldn't help me understand why. Also, the serial card being unreliable, while also fixing the onboard with identical schematic, makes me thing this won't work, because that second serial card, is now right next to the bus/CPU area.

  8. #18

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    how noisy is the bus? have you put an oscope on it? com port requires IRQs. wonder if its a bus timing issue.

  9. #19

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    Quote Originally Posted by BloodyCactus View Post
    how noisy is the bus? have you put an oscope on it? com port requires IRQs. wonder if its a bus timing issue.
    I'll check with a scope tonight. Maybe compare it to a 5150 with an identical serial card?
    D7, IRQ4, A3 ?

  10. #20

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    Quote Originally Posted by BloodyCactus View Post
    how noisy is the bus? have you put an oscope on it? com port requires IRQs. wonder if its a bus timing issue.
    What should I probe specifically to find a noisy bus?

    I've checked some things.

    > The UART stops generating IRQs whenever the mouse drops out.
    > The UART CS line is always active. This makes sense since it will go low whenever 3F8 is on the address bus, regardless of any address lines above A9. Forgive my noob question but I've never figured that part out regarding chip-selects on ISA. If the CS logic is not looking at the higher address lines, wouldn't all the false chip-selects cause issues? Or is this related to IOR/IOW? IOR/IOW can only be active with addresses within the first 10 address lines?

    IBM 5150 - A7:
    5150 A7.jpg

    NuXT - A7:
    NuXT A7.jpg

    NuXT - UART CS:
    NuXT UART CS.jpg

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