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

  1. #1
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    Unhappy IBM 5150: VGA boots to BASIC, but CGA won't?

    Although it runs fine with my late '90s VGA card and display, I was recently able to get a more period-correct IBM 5153 display and Color Graphics adapter card for my IBM 5150. My hard card is malfunctioning, and I'm expecting to get to IBM Cassette BASIC, but in CGA mode, despite changing jumper settings, it POSTs properly, but after that, the fat boot cursor switches to a thinner white blinking cursor that reacts to key presses, but won't show any characters on the screen. Any idea what is going on? I don't actually have anything to boot to from floppy to test anything else right now.

    I'm guessing that I am in BASIC, but not seeing anything there? Anyone have any ideas? Thanks.

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    Are you DIP switches set properly for 80x25 color? That's required for CGA.

    The DIP switches block for the 5150 are 40x25 color, 80x25 color, MDA, and "other". Yours is likely set to "other" for the VGA. www.minuszerodegrees.net can help.
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    Try doing something that would beep the speaker, or click the cassette relay. Or perhaps blindly type commands in that would print a stream of characters (something like: for i=1 to 9999 : print "hello" : next) and see if the cursor moves on its own. If that happens, that sounds like a fault in the CGA card such as the character generator ROM.

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    Welcome to these forums.

    Quote Originally Posted by RootHouston View Post
    I was recently able to get a more period-correct IBM 5153 display and Color Graphics adapter card for my IBM 5150.
    Always good to know what we are working with.

    Did the supplier indicate that the items are fully functional ?

    Quote Originally Posted by RootHouston View Post
    ... but in CGA mode, ...
    This needs expansion/clarification. Do you mean that the motherboard's two video switches are set for CGA ?

    Quote Originally Posted by RootHouston View Post
    ... despite changing jumper settings, ...
    What jumpers are those? The IBM CGA card has no jumpers, and the 5150 motherboard has only one jumper which is a setting for the cassette port.

    Quote Originally Posted by RootHouston View Post
    ... it POSTs properly, ...
    What do you see/hear that makes you believe that?

    Quote Originally Posted by RootHouston View Post
    ... that reacts to key presses, but won't show any characters on the screen.
    So, the curcur simply moves one position to the right for each key press?

    Quote Originally Posted by RootHouston View Post
    I'm guessing that I am in BASIC, but not seeing anything there?
    Try typing in the following BASIC command, then press the ENTER key.
    If you are in BASIC, that command will beep the speaker.

    PRINT CHR$(7)

    Quote Originally Posted by RootHouston View Post
    Anyone have any ideas?
    VGA card removed?

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    Thanks everyone for responding to a newcomer. Just to clear-up some things:

    • I'd mistakenly called the DIP switches as "jumper settings". Forgive me.
    • The DIP switch settings are, in fact, configured for CGA cards. I have the 5 switch on, and the 6 switch off when the CGA card connected. I've included a photo here later in the post. For VGA, I use both 5 and 6 on. I am not attempting to use them simultaneously.
    • Trying BASIC commands was a great suggestion! It seems that I am definitely in BASIC, as performing the speaker beep and for loop both worked to prove that's where we are.
    • I've tried this card using both the 40x25 and the 80x25 color modes in the DIP switch settings. Swapping between them only changes the cursor sizes for me. Unfortunately, I still see no characters.


    The company that I purchased the card from claimed that their items are believed to be in working condition, but this one in particular hadn't been tested by them. They have a return policy, that I guess I might have to exercise. It's too bad, as this card looks great physically. It arrived with no dust, and is free of any corrosion or damage on the chips. This is actually my second attempt to get a CGA card. The first time, the seller mislabeled the item, and sent me an MDA card instead. With this second seller, I get this issue.

    Here are my jumper settings for CGA mode:


    Here's a photo of the actual card:


    Here are my questions now:
    • Are these cards very prone to failure? Not sure if I should be trying a modern, third-party solution instead
    • What is the probability that this is an issue with the CGA display instead?
    • Anything else I should try with this card before hanging it up?

    Thanks to everyone.

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    I've never seen a CGA card failure where it displayed absolutely nothing; usually some bits of RAM are bad but never nothing at all.

    If you see a blinking cursor, and a wider cursor when you switch to 40x25, then your monitor is working at a bare minimum of functionality with which to troubleshoot the CGA card and the rest of the system.

    I would try to get yet another CGA card to see if that works, preferably a known working one.
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    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.
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    The cursor is indeed hardware, however it's generated by the motorola 6845, and that's the character generator.
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    Quote Originally Posted by Trixter View Post
    The cursor is indeed hardware, however it's generated by the motorola 6845, and that's the character generator.
    ... are you sure about that? The 6845 is a CRTC, "CRT Controller", that generates the timing signals, memory address, and row offsets for the attached video generation hardware. The "character generator" is a 9264 ROM chip. The CRTC generates the address bits, and then the CGA card has a serializer section that takes output either directly from some latches in front of the RAM or indirectly from RAM contents run through the generator ROM and generates the actual video signal sent to the monitor.

    Skimming the CGA schematics and the 6845 CRTC datasheet there's a "CURSOR" output on the CRTC that "indicates a valid cursor address to external video processing logic". It looks like this output goes to a gate that adds a tiny delay, which then goes to a horrible mess of gates that generates a signal called "alpha dots"(*), and then *that* disappears into a web of circuitry that makes my eyes bleed. My guess is that this is part of the serializer that allows this "cursor active" signal to be translated into a line/block of dots which is either overlaid or not over the character generator output depending on the status of the "blink" toggle. If that's all true then, well, if the serializer, RAM, or character generator ROM were broken in such a way that the other inputs to the serializer just looked like a bunch of black dots wouldn't that explain how you could have a cursor being painted but nothing else?

    EDIT: I looked again at the inputs to that "Alpha Dots" section, and one of them is the serialized output from the 74LS166 shift register that's sitting on the output of the character generator ROM. It also takes cursor blink *and* attribute blink signals, so basically it's a catch-all for what dots need to be combined with the colors generated in the color attribute section. (which I now think is the really eye-bleedy part) So, yeah, if, for instance, that 74LS166 shift register were spitting out nothing but dark pixels that would totally explain why there's a cursor but nothing else.

    I'd actually guess it's about a coin flip whether the graphics mode will spit out interesting output or not?

    ... another thing that might be interesting to try at the BASIC prompt: issue a COLOR statement that sets the background to something other than black.
    Last edited by Eudimorphodon; September 14th, 2020 at 03:19 PM.
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    Apologies for using incorrect terminology, and yes, I suppose it is possible. But in all my years handling 30+ CGA cards, I've never seen a failure like that. Still, anything is possible, which is why a known good working CGA card is required for the OP to continue diagnosing through process of elimination.
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