Re: Dead PET 2001 with 6550´er

From: Nicolas Welte (
Date: 2002-04-16 15:46:09

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    Marko Mäkelä wrote:
    > On Tue, 16 Apr 2002, Nicolas Welte wrote:
    > > It seems the VSYNC signal is also needed for the computer part to
    > > start up (IRQ source, IIRC), so this machine also showed the dead
    > > machine symptoms.
    > True, VSYNC is the source of the periodic system interrupt that reads the
    > keyboard and updates TI.  I don't know what other things it could do.  But
    > shouldn't the PET display the start-up screen when this IRQ source is
    > disabled?  It just wouldn't respond to the keyboard.
    I think it didn't show any start-up screen, because I had a multisync monitor
    attached, via my userport-to-DSUB adaptor. There I should've seen a rolling
    start-up screen, but I only got an empty raster.
    Doesn't the same happen to the C64 as well, if you remove the IRQ-CIA, it
    starts up with an empty screen, with the correct colors? I know for sure it
    happens to the CBM 710 if you don't supply the 50Hz signal from the
    > I think that there must have been other problems as well in that machine.
    > Maybe the ROM code hung in some place where it polled the state of the
    > video circuitry to see when data can be safely written on the screen
    > (remember the "smoke poke" discussions a few years ago?).
    Yes, the screen output routine waits for the vertical refresh so it doesn't
    interfere with the video output. Could explain the behaviour very well. But
    why does it also happen on the C64 and the 710? I don't think those wait for
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