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

  1. #21
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    Egads! That's nasty! It's like my logo but IRL.

    Qualify your address decoding.

  2. #22
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    Just a thought--what do your power rails look like (waveforms) near the troublesome chips?

  3. #23

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    Quote Originally Posted by Chuck(G) View Post
    Just a thought--what do your power rails look like (waveforms) near the troublesome chips?
    Here's the 5V rail at various places on the board, with both an awful PSU and a decent PSU.

    Attached Images Attached Images

  4. #24
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    Quote Originally Posted by nztdm View Post
    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:
    Attachment 52393

    NuXT - A7:
    Attachment 52394

    NuXT - UART CS:
    Attachment 52395
    Would those spikes be solved by installing those termination resistor packs on the end of the bus?
    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. #25

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    Quote Originally Posted by blackepyon View Post
    Would those spikes be solved by installing those termination resistor packs on the end of the bus?
    Nope I tried that.
    And they may have to be on the other side of the PCB by the serial circuitry.

    The spikes also seem to be in a working 5150 so I can't tell if they're bad or not.
    I'm having an email conversation with glitch to try figure it out.

  6. #26

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    The NuXT Rev1.1 is ready. Uploaded to GitHub.

    I have built and tested one, with as many cards and software as I can, and I can't get the serial port to drop out. It passes the CheckIt Serial Port test now. With the Rev1.0 it would always fail on "Modem status", and sometimes other sections.

  7. #27

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    Quote Originally Posted by nztdm View Post
    The NuXT Rev1.1 is ready. Uploaded to GitHub.

    I have built and tested one, with as many cards and software as I can, and I can't get the serial port to drop out. It passes the CheckIt Serial Port test now. With the Rev1.0 it would always fail on "Modem status", and sometimes other sections.
    nice work! What was the fix for the noisy bus?

  8. #28

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    Quote Originally Posted by BloodyCactus View Post
    nice work! What was the fix for the noisy bus?
    Was there a noisy bus? I couldn't tell.

    But moving the serial circuitry close to the bus rather than on the far end of the board, fixed it.
    I decided to try this after I was able to reproduce the problem on a backplane and individual cards, when the identical-schematic'd serial card was at the far end of the backplane from the CPU card.

  9. #29
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    Quote Originally Posted by nztdm View Post
    Was there a noisy bus? I couldn't tell.

    But moving the serial circuitry close to the bus rather than on the far end of the board, fixed it.
    I decided to try this after I was able to reproduce the problem on a backplane and individual cards, when the identical-schematic'd serial card was at the far end of the backplane from the CPU card.
    I had a similar problem with a card I'm working on. Had to make sure the 245 data buffer was closer to the bus, otherwise I ran into intermittent issues. Moving the 245 closer fixed them.
    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.

  10. #30

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    This one has the 16550 connected to the bus directly. (The bus itself is goes through a 245 from the CPU, but that's to separate AD bus and D bus, and maybe for driving it if the CPU/chipset is too weak; I am ignorant here.)

    I tried on my Rev1.0 bodge board to give the 16550 its own 245, to no effect.

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