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Thread: Trident VGA and IBM PC

  1. #51
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    Quote Originally Posted by Vic3Dexe View Post
    You can extend ALE to the front of /MRD signal (as Tronix did).
    Or just use front of /MRD to latch address - I think this will also work.
    I implemented an easier approach - it only uses ALE and AEN signals.
    I believe this approach is also more reliable. It doesn't change ALE (other than adding a slight delay) when AEN is inactive, and therefore it will not break AT systems. (Most likely Tronix's schematic will not work on an AT in 16-bit slot, because LA17-LA23 will not be valid when /MEMR or /MEMW are activated).

  2. #52

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    Some people reported ultra light solution - just pull up TVGA9000 ALE pin to +5V....

  3. #53

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    Quote Originally Posted by Tronix View Post
    Some people reported ultra light solution - just pull up TVGA9000 ALE pin to +5V....
    Cool! I assume you do this with a resistor as always?

    This trick would be useful to get this working on any XT I have in the future; but does the TVGA8900D/9000 chipset have any compatibility with the Olivetti M24's byteswapped BIU implementation? I fear it's what breaks most "XT compatible" video cards from VGA and up on this machine.
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  4. #54
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    Quote Originally Posted by Tronix View Post
    Some people reported ultra light solution - just pull up TVGA9000 ALE pin to +5V....
    Huh, tested this on the boards I have available: IBM PC, IBM XT, PC-Retro, a chipset-based Turbo XT, 286-AT, and it works perfectly.
    Apparently Trident TVGA has transparent latches on the address lines, tying ALE pin high keeps these latches in "transparent" mode, and it works as long as the address remains valid during I/O or memory operation.

    That's indeed a very easy fix.

  5. #55
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    Quote Originally Posted by Kazblox View Post
    Cool! I assume you do this with a resistor as always?
    Resistor is not needed (but won't hurt), just disconnect TVGA ALE pin from ISA, and connect it to +5V.

    does the TVGA8900D/9000 chipset have any compatibility with the Olivetti M24's byteswapped BIU implementation? I fear it's what breaks most "XT compatible" video cards from VGA and up on this machine.
    I don't think it is something that a VGA chipset could possibly fix it. The problem is that VGA registers are accessed by writing register index to a lower byte of a 16-bit port and value to the higher byte. If bytes are swapped, it will write the value first... (likely to the wrong register)
    Last edited by sergey; November 16th, 2017 at 09:22 AM.

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