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Thread: CGA card issue (freeze)

  1. #1
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    Default CGA card issue (freeze)

    Hi,

    I am currently restoring an IBM PC XT 5160 (8088 ).

    I solved the problems of access to the hard disk by low-level formatting, but I encounter a strange issue with the CGA graphics card: the display is correct until the text reaches the last line of the screen (ex: dir C: ). Then the display freezes but the cursor continues to flash, instead of scrolling.

    I replaced the CGA card with a VGA card (Cirrus Logic 5401 16-bit ISA) and the display works correctly.
    I would like to diagnose and repair the CGA card but visually, everything looks correct.

    If you have already encountered this problem, thank you in advance for your help.

    Thierry (from France)

  2. #2
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    Hello. Welcome to these forums.

    I suggest that you run some diagnostic software against the card. CheckIt is an example (from the main menu, [TESTS] then [VIDEO]).

  3. #3
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    A defect of this kind might be a defective card, but honestly it sounds like the problem is software-related. Is your CGA card a true IBM CGA card, or a clone? What DOS are you booting? Are you loading any TSRs or other programs in your CONFIG.SYS or AUTOEXEC.BAT?
    Offering a bounty for:
    - The software "Overhead Express" (doesn't have to be original, can be a copy)
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  4. #4
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    Quote Originally Posted by Trixter View Post
    A defect of this kind might be a defective card, but honestly it sounds like the problem is software-related. Is your CGA card a true IBM CGA card, or a clone? What DOS are you booting? Are you loading any TSRs or other programs in your CONFIG.SYS or AUTOEXEC.BAT?
    It's IBM CGA Card and I booting with same DOS disk to test VGA and CGA cards.

    Motherboard's switchs seems OK : 5=ON , 6=OFF : CGA, at 80 column by 25 line mode

  5. #5
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    Quote Originally Posted by modem7 View Post
    Hello. Welcome to these forums.

    I suggest that you run some diagnostic software against the card. CheckIt is an example (from the main menu, [TESTS] then [VIDEO]).
    Thank for this trick, when I can redo floppies 5 1/4 I will do the test

  6. #6
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    If I remember correctly, the CGA BIOS routines wait to do the memory shuffle for scrolling until there's a vertical blanking interval to avoid "snow" on the display. If your CGA card isn't correctly indicating the retrace, that would cause things to behave the way you describe.

  7. #7
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    Quote Originally Posted by Chuck(G) View Post
    If I remember correctly, the CGA BIOS routines wait to do the memory shuffle for scrolling until there's a vertical blanking interval to avoid "snow" on the display. If your CGA card isn't correctly indicating the retrace, that would cause things to behave the way you describe.

    This is an interesting track: I have just visually checked the connectors of the CGA card and the screen (IBM 5153), but I do not see anything particular. But can be a problem of false contact? oxidation?

  8. #8
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    Quote Originally Posted by ZeroG1972 View Post
    booting with same DOS disk to test VGA and CGA cards
    Please post the contents of your CONFIG.SYS and AUTOEXEC.BAT from this disk, just to eliminate software as the culprit.
    Offering a bounty for:
    - The software "Overhead Express" (doesn't have to be original, can be a copy)
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  9. #9
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    Quote Originally Posted by ZeroG1972 View Post
    This is an interesting track: I have just visually checked the connectors of the CGA card and the screen (IBM 5153), but I do not see anything particular. But can be a problem of false contact? oxidation?
    Could be that vertical retrace simply isn't generated on this card because of a bad IC. Probably time for a logic probe or a bit of programming.

  10. #10

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    I don't recall, but does the 5160 BIOS check for retrace even in 40-column mode? After all there should be no snow problem there.

    If it doesn't, that could help test for this specific problem at least.
    int10h.org :: :: :: blog

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