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

  1. #21

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    Could it be that the system still sees the dip switches are set for MDA? Wouldnít that cause a blinking cursor on the CGA monitor? I donít have a 5150 to test and my 5160 isnít easily accessible at this exact moment, but I do recall my 5160 having issues with the switches not reading correctly until I flipped them back and forth a bit.

  2. #22
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    Quote Originally Posted by modem7 View Post
    Entering "key off" removed the black blocks, and entering "color 4" showed a red block.
    Photo at [here].
    Yay. So that shows the attribute circuitry works as expected with the character generator pulled out. (If you ran "CLS" after the color command your whole screen should turn red.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  3. #23
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    Quote Originally Posted by jafir View Post
    Could it be that the system still sees the dip switches are set for MDA? Wouldnít that cause a blinking cursor on the CGA monitor? I donít have a 5150 to test and my 5160 isnít easily accessible at this exact moment, but I do recall my 5160 having issues with the switches not reading correctly until I flipped them back and forth a bit.
    Not a bad idea, but it does currently work when I set the switches to VGA, so that is probably out.

  4. #24
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    Quote Originally Posted by RootHouston View Post
    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.
    Ugh, I'm sorry your reply took so long to show up. You'll need to post a few times before you get out of moderator molasses territory. (10 posts total.)

    So.... that's interesting! The bottom half of your screen is correct, but why in the world is the top half of your screen a mirror of it? That actually makes me think that maybe the problem *isn't* the character generator, but something going on with memory addressing.

    I know typing this blind is going to be a pain, but can you try this immediately after dropping into BASIC?

    10 def seg=&hb800 : for x=0 to 16383 : poke x,111 : next x
    run

    It'll take a while to run. (Around 30 seconds.) If my latest theory is correct, at some point in the middle of that runtime your screen will fill with white "o"'s on an orange background. Start a stopwatch and keep track of when those "o"s appear, if they do, and the total time before you see a blinking cursor again.

    I think you may have a stuck address line.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  5. #25
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    Quote Originally Posted by Eudimorphodon View Post
    (If you ran "CLS" after the color command your whole screen should turn red.)
    It does.

  6. #26
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    Just a heads-up, you'll want to look back through responses here, since the approval stuff is putting my responses out of order now. To recap. I've done the screen 2 trick and the re-seating/replacement of the character ROM now to no avail.

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    Quote Originally Posted by jafir View Post
    Could it be that the system still sees the dip switches are set for MDA? Wouldnít that cause a blinking cursor on the CGA monitor?
    I did some experiments a while back. The results are at [here].

  8. #28
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    Quote Originally Posted by RootHouston View Post
    Just a heads-up, you'll want to look back through responses here, since the approval stuff is putting my responses out of order now. To recap. I've done the screen 2 trick and the re-seating/replacement of the character ROM now to no avail.
    Yeah. It's annoying that's how it works, but I guess you have to keep the spammer riffraff at bay somehow.

    You can try my latest suggestion and see if it does something. My harebrained theory is that *either* writes from the computer are going to the wrong place in video memory (problem on the "in" side), or the address generation on the card is screwed up and outputting the wrong areas of memory. As an amendment to it: you can also try the same poke loop in graphics mode (screen 2), it should fill the screen with regular dots and dashes. What you'll *probably* see is *nothing* for a while, then you'll see a pattern windowshade in on both the top and bottom halves of the screen for a while, then nothing, then the gaps get filled in. My theory is that the video display hardware is doing something like skipping the first and third 4K of memory, instead displaying the second and fourth 4K twice. If that's the case, and *writes* are still going to the correct places, then that's why you just see a cursor drawn over nothing in text mode *and* the BASIC graphics mode contents are doubled.

    If this is it it could be a bad CRTC chip.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  9. #29
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    Quote Originally Posted by Eudimorphodon View Post
    Ugh, I'm sorry your reply took so long to show up. You'll need to post a few times before you get out of moderator molasses territory. (10 posts total.)

    So.... that's interesting! The bottom half of your screen is correct, but why in the world is the top half of your screen a mirror of it? That actually makes me think that maybe the problem *isn't* the character generator, but something going on with memory addressing.

    I know typing this blind is going to be a pain, but can you try this immediately after dropping into BASIC?

    10 def seg=&hb800 : for x=0 to 16383 : poke x,111 : next x
    run

    It'll take a while to run. (Around 30 seconds.) If my latest theory is correct, at some point in the middle of that runtime your screen will fill with white "o"'s on an orange background. Start a stopwatch and keep track of when those "o"s appear, if they do, and the total time before you see a blinking cursor again.

    I think you may have a stuck address line.


    Okay, so it definitely brought-up some white 0s with an orange background. Not sure if they were ever supposed to disappear, however, the blinking cursor did appear overlayed on top of it all after about 41 seconds of seeing the characters start to appear. See my video above. Characters start at about 13 seconds in, and the blinking cursor appears again at about the 54 second mark somewhere in the top half of the screen. Might be a bit difficult to see the blinking cursor in the video, but if you look closely, it's there. No need to watch the whole thing. I left it recording for another another 9 minutes to see if anything else would happen, but it didn't. Even after 40 minutes+ the characters never disappeared in the background.

    Does this confirm your theory?

  10. #30
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    Just wanted to also report that I bought a different CGA card, and it seems to work fine. So, it's definitely an issue with the card itself.

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