Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Back in Action with Cumulus!

Posted by retromaster on July 28, 2011

After a few months of absence due to some personal issues, I am glad to announce that for now, everything seems to be back on track :). Now a status update on Cumulus.

Some of the close followers of the Cumulus project may already be aware that, during testing some months ago, some Oric machines failed to boot properly with Cumulus. Lately, I’ve had the chance to work on this issue. It turns out that, due to the notoriously poor clock signal of the Orics (almost sawtooth instead of square!), the bus timings were off with the machines that failed. I have managed to get the failing machines to boot, but there are still timing issues, especially with the MAP signal, as one can observe video corruption from time to time.

I am still working on a solution to this problem… One possible solution is to increase the 32Mhz clock on the Cumulus to 64Mhz. This would enable more accurate sampling of the incoming clock signal and therefor improve MAP signal timing. I do not have any 64Mhz clock oscillators handy though, so I’ll have to order them. I am also not ruling out the possibility that there may be some other signal integrity issue at play here. More tests will probably point this out.


8 Responses to “Back in Action with Cumulus!”

  1. Sounds good retro .. I am surprised that I didn’t observe this timing problem with one of my Atmos machines yet .. I will dig up the last two (I have quite a few now) and see if I can find a faulty condition to test with – that way if you come up with a fix I can help you verify it.

  2. retromaster said

    Please do test on as many machines as you can, Jay. I’ve so far tested on 3 Atmos machines, only 1 works perfectly, the others have video/bus issues. I also have an Oric-1 that I’ve never used, if that machine works, I’ll be testing with that one, too.

  3. retromaster said

    And Jay, I have a question for you… Are you using Cumulus with the short end of the IDE cable or the long end (gray connector or blue connector)?

  4. Symoon said

    I have about 5 Atmos and 2 Oric-1. Exprienced problems with the Atmos, trying to load tapes while the disk drive was plugged, which it seems is due to a poor signal. The Oric-1 were fine!
    I am absolutely no hardware expert but if you ever need further testing, let me know.

  5. What I wonder is if its related to power supply? I have my 8-bit workbench set up with pretty good power these days, I’m using a beefy PSU. Also, my Atmos’ have been hardly ever used, I bought them as a lot.

    Incidentally, I have the weekend for testing and hacking reserved, so I’ll get more details to you on mail as I set things up. 🙂

  6. retromaster said

    I did some more testing and found out that the PH2 clock signal from the Oric is more “glitchy” on the failing machines. This might be caused by crosstalk from another signal, but why it happens on some machines and not on others is beyond me.

    Anyway, these glitches are partly more problematic than they should be, because the buffers on the CumuluBus are 74ACT541 types. Next week I’ll try to obtain some 74HC541s to replace them. These parts are slower and have a higher minimum high input voltage level (3V instead of 1.5V), so perhaps this will fix the issue.

    In the light of this info, I think the PSU is unlikely to be the cause, but it would be good to test and rule it out for sure.

  7. Dbug said

    If you need a larger test sample, I have two Atmos, one Oric 1, and one Pravetz 8D as well. I also have a Telestrat, but I’m not sure Cumulus would work on it? (or perhaps with the Romatmos cartridge).
    Typically my Microdisc unit works fine on the Pravetz, but never worked with any of the three other Orics 🙂

  8. retromaster said

    I’ll first try to get the machines I have working, then we can proceed with a larger test population 🙂 And as you guessed, Cumulus is not likely to work with a Telestrat, but for a Telestrat, better solutions exist, don’t they? 😉

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