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Thread: IBM 5150: VGA boots to BASIC, but CGA won't?

  1. #11
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    It almost seems like it has to be something broken in the memory-character generator-serializer chain. If the PC were mis-detecting the card it seems unlikely you’d get a clear cursor, because the CRTC has to be programmed to valid timings on startup. (And it’s honoring the 40/80 column switch.) Still really curious if the graphics mode works, because that’s a mostly separate hardware chain on the CGA card.
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  2. #12
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    Quote Originally Posted by Eudimorphodon View Post
    Does the CGA card have a hardware cursor (IE, is the little blinking underline generated directly by the CRTC)? I *think* it does, and if so then theoretically there could be something up in the character generator/RAM circuitry that would prevent it from being able to display characters while the blinking cursor survives.

    Just for laughs, what happens if you blindly type "screen 2" after it ends up in BASIC? That will switch it from text to 640x200 graphics, it's possible if the issue is with, say, the character generator the card will still be able to generate bitmaps.
    You are correct. It has the little blinking underline. Your suggestion was very interesting! I ended-up getting a screen with some text there that resembles BASIC in a weird way. I took a quick screenshot with my phone. Sorry for the quality. My cell phone doesn't seem to want to focus very well on these large dots. I do have another CGA card that is supposed to show-up this Friday. I guess we're safe to assume it's the character ROM now? Let me know if there are any other things you'd like me to test in the interim.


  3. #13
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    Quote Originally Posted by Eudimorphodon View Post
    Still really curious if the graphics mode works, because thatís a mostly separate hardware chain on the CGA card.
    Supporting that:

    I removed the hard disk controller from my IBM 5160 so that the 5160 would boot to Cassette BASIC.
    On the 5160's IBM CGA card, I removed the character ROM from its socket.
    I then powered on the 5160.
    I saw a white raster.
    After the POST completed, the screen changed to that shown at [here].
    After I entered "screen 2", the screen changed to that shown at [here].

  4. #14
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    * Are these cards very prone to failure? Not sure if I should be trying a modern, third-party solution instead
    Not any more prone than any other card.

    * What is the probability that this is an issue with the CGA display instead?
    You mean the monitor? I'd say not likely.

    * Anything else I should try with this card before hanging it up?
    Since the character ROM is socketed, try carefully removing it, checking the legs and the socket, then re-seat it.
    Also, try the "screen 2" command as Modem 7 suggested, if that displays something sane then that at least proves the video RAM is working fine. If it displays anything garbled, post a photo.
    Whatever you do don't toss the card, I'm sure someone could repair it. If it happens to be just the character ROM, that would be an easy fix.

  5. #15
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    Quote Originally Posted by modem7 View Post
    I saw a white raster.
    After the POST completed, the screen changed to that shown at [here].
    I was going to ask if you were able to see a blinking cursor, but I just verified that if the default underline cursor is over a character where there's white on the 8th line (like the solid block graphics character) then it doesn't XOR the existing dots during the blink, the cursor just disappears. So it'd be expected behavior not to see it.

    Anyway, yeah, that certainly demonstrates the idea. You're getting white pixels instead of black because TTL logic inputs usually float high if they're not tied to something, so the 74166 shift register that sits on the character generator output is seeing all ones with it pulled out. If it is specifically the character generator that's blown on the OP's card it's messed up in some way that's making it output solid zeros instead.

    If you still have the the thing pulled apart I'd be interested to see confirmation that issuing a "color" command would change the color of the raster. (Also, "key off" should get rid of the blank areas at the bottom...) With your card spitting out all "1"'s in the dot generation loop you'll probably only see changes to the foreground color, but with the OP's card if the problem is isolated to this area I'd expect issuing, say, "COLOR 4,1" should produce a red cursor on a blue background. (A "CLS" will fill the screen with the associated color.)

    After I entered "screen 2", the screen changed to that shown at [here].
    Woot.

    So, yeah, if the OP's card *does* work in graphics mode that isolates the problem area a *lot*. In the chain between RAM and "alpha dots" there's a 74LS374 (?, from memory) latch, the character generator, the 74LS166 shift register, and the chain of gates in the "alpha dots" circuitry itself. (Since we see the cursor itself I'd expect the stuff downstream from Alpha Dots to be okay.) If graphics mode works it almost certainly limits the problem to that region, or *possibly* one of the clock/enable lines for one of those components.

    (If graphics mode *doesn't* work and/or that test of trying to set color attributes fails then we're looking at a more general problem, like the video memory or possibly some brokenness in the support circuitry, like address generation or read pulse timing.)
    Last edited by Eudimorphodon; September 15th, 2020 at 08:17 AM.
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  6. #16
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    I apologize for the disjointed replies, but since I'm new here, my posts are still being manually moderated and approved for posting. I am 35 years old now, and have never removed a socketed chip in my life, but with the help of YouTube, I got confident enough to try to reseat the character ROM chip as per a suggestion from Eudimorphodon. Unfortunately, it didn't have any effect. I then read here that the MDA cards used the same character ROM as the CGA, and realized that I had 2 of those lying around. The first came with my machine. The second one, as I may have mentioned previously, was sent to me in error.

    Now, I'm not certain if the 6359300 and 5788005 chips are the same, whereas the CGA card had the 6359300 chip, and the MDAs both had the 5788005 chip, I swapped them anyway. Unfortunately, even swapping for the 5788005s had the same result. Unless I just have real bad luck or the 5788005 chip is just not compatible on this card, I'm guessing that the chip isn't necessarily malfunctioning, and something else is causing it to not work.

  7. #17
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    ... actually, one thing does occur to me, since the OP never provided a picture of the screen. If it's *just* the dot generation that's broken and the condition is it's spitting out all "black" dots then, if everything else is working properly, in addition to the grey cursor they should also be seeing some gray blobs down at the bottom of the screen where the reversed-video function key labels are in BASIC. If they're not seeing those it might already be a tipoff of bigger problems. (Like RAM being broken so the writes to set the color attributes in those screen locations to reverse video went nowhere or otherwise can't take effect.)
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  8. #18
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    Quote Originally Posted by modem7 View Post
    After I entered "screen 2", the screen changed to that shown at [here].
    That's brilliant; wish I'd thought of that. Hope the OP tries that.
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  9. #19
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    Quote Originally Posted by SomeGuy View Post
    Also, try the "screen 2" command as Modem 7 suggested, ...
    Suggested by Eudimorphodon at post #7

    Quote Originally Posted by Eudimorphodon View Post
    If you still have the the thing pulled apart I'd be interested to see confirmation that issuing a "color" command would change the color of the raster. (Also, "key off" should get rid of the blank areas at the bottom...)
    I will get onto that then report back.

  10. #20
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    Quote Originally Posted by Eudimorphodon View Post
    If you still have the the thing pulled apart I'd be interested to see confirmation that issuing a "color" command would change the color of the raster. (Also, "key off" should get rid of the blank areas at the bottom...)
    Entering "key off" removed the black blocks, and entering "color 4" showed a red block.
    Photo at [here].

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