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Thread: Storage and 25 MHz options for 286

  1. #1

    Default Storage and 25 MHz options for 286

    Hi

    I have built a V30-based Olivetti M240 (sry for Estonian language - https://foorum.hinnavaatlus.ee/viewtopic.php?t=682720) and saw that my thirst for building retro computers only got worse.

    While researching for 286, I accidentally got hold of Harris 25 Mhz CPU and was now looking for motherboard. All in all, this seemed like a nice fit - http://www.uncreativelabs.de/th99/m/A-B/33419.htm -- offering 25 Mhz and 16 MB RAM. And as luck would have it, I am now in possession of both.

    However, seeing that the board comes equipped with 32 and 40 MHz crystals and Harris 20 MHz CPU, I fail to understand how does it support 25 MHz CPU. Am I missing something? I probably can crystal mod it, but the whole idea of my builds have been to see how fast things go per factory specs. Basically drop-in replacements and don't really wan't to go into overclocking. So is this really a 25 MHz board? Or were people expected to swap out the crystals back then? And also, why does this board have two of them?

    Another question I had was related to storage. The choice for 086 was obvious, as it only had 8-bit ISA slots -- I used lotech XT-CF card together with IDE-CF adapter. However, since 286 has 16-bit ISA slots, I'm not convinced that this 8-bit XT-CF is the best option. I have a 16-bit IO controller and had an idea to install the IDE-CF adapter together with it. The idea was to use the 16-bit IO controller but have the XTIDE BIOS to find the CF card for me. However, it did not work. Mounting the IDE-CF adapter to XT-IDE worked like it should, but while mounting the IDE-CF to the IO card XTIDE BIOS no longer finds the CF. So the question really is - how to get modern CF cards working on a 16-bit ISA bus? Or should I even care and use the 8-bit XT-CF?

  2. #2
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    Quote Originally Posted by tonism View Post
    However, seeing that the board comes equipped with 32 and 40 MHz crystals and Harris 20 MHz CPU, I fail to understand how does it support 25 MHz CPU. Am I missing something? I probably can crystal mod it, but the whole idea of my builds have been to see how fast things go per factory specs. Basically drop-in replacements and don't really want to go into overclocking. So is this really a 25 MHz board? Or were people expected to swap out the crystals back then? And also, why does this board have two of them?
    Is the 40 MHz crystal in a socket ?

    I think there must be a jumper somewhere which selects which oscillator is used to clock the CPU.
    Something like:

    16 MHz CPU operation: Jumper in position X, to select the 32 MHz oscillator.
    20 MHz CPU operation: Jumper in position Y, to select the 40/50 MHz oscillator. 40 MHz oscillator fitted in socket.
    25 MHz CPU operation: Jumper in position Y, to select the 40/50 MHz oscillator. 50 MHz oscillator fitted in socket.

    32 MHz oscillator divided by 4 to get an 8 MHz clock for the ISA bus.

    Quote Originally Posted by tonism View Post
    ... and don't really want to go into overclocking.
    Any of the motherboard's chips rated at 20 MHz ?

  3. #3

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    Yes, people were in fact expected to swap xtal oscillators with CPUs back then. The second one is probably for a math co processor, I don't think there are any 287s that will run above 16MHz

  4. #4

    Default

    As for storage, I did a thread about the best options for a fast 286 a while back. Tl;dr 10kRPM SCSI is the best with a bus mastering HBA

    http://www.vcfed.org/forum/showthrea...-a-286-Machine

  5. #5

    Default

    Quote Originally Posted by modem7 View Post
    Is the 40 MHz crystal in a socket ?
    ...
    Any of the motherboard's chips rated at 20 MHz ?
    Quote Originally Posted by maxtherabbit View Post
    Yes, people were in fact expected to swap xtal oscillators with CPUs back then. The second one is probably for a math co processor, I don't think there are any 287s that will run above 16MHz
    The crystals both are soldered so its little hard to believe that this board was designed so one would need to solder-replace the crystal.

    I tried AMD 287-10 as FPU, but it was too unstable. Makes sense if the second crystal is for that.

    The board itself has VLSI VL82C311L chipset. I did some digging and found this - https://www.datasheetarchive.com/pdf...term=VL82C311L
    I think the confusion is that while the chipset is rated for 25 Mhz, its only for 386. For 286 it goes up to 20 MHz and that's how Biostar made it as well.

    My 25 MHz Harris is looking rather sad now
    https://drive.google.com/uc?id=1RhEK...Qb3N3BFw07I23Z
    Any ideas or hints where to find 25 MHz decent board? Its a real shame since that Biostar board seems to do so much. 16 MB RAM, video and adapter shadowing, EMS, memory timings, clock dividers for different modes.. I'm not that deep in the retro PC BIOS options, but that looks rich to my eyes.

    Quote Originally Posted by maxtherabbit View Post
    As for storage, I did a thread about the best options for a fast 286 a while back. Tl;dr 10kRPM SCSI is the best with a bus mastering HBA
    http://www.vcfed.org/forum/showthrea...-a-286-Machine
    That's a good read, thank you! So, any hints on a good 16-bit SCSI adapter and HDD?

    I'm using XT-CF on my V30-based 086 - perhaps I should look into SCSI there as well? I should have one 8-bit SCSI adapter somewhere.

  6. #6
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    Call me crazy but I thought the fastest production 286 was the Harris 20mhz (where did the 25Mhz come from?).

  7. #7
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    I have seen 25MHz 286s back in the 90s, so they're out there.
    But I don't think there's a way to run the system at zero-wait, so that kind of defeats the purpose of a fast processor.

  8. #8

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    Quote Originally Posted by dorkbert View Post
    I have seen 25MHz 286s back in the 90s, so they're out there.
    But I don't think there's a way to run the system at zero-wait, so that kind of defeats the purpose of a fast processor.
    I'm currently running that 20 MHz with 0WS. Board came with 70ns DIP20 RAM if I'm not mistaken and going strong (only conventional memory tho). Sadly my 4x 1MB 70ns SIP30 SIMMs did not work. I'm thinking of ordering 4x 4MB 60ns SIMMs. Maybe even some 50ns ones.

  9. #9

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    Quote Originally Posted by tonism View Post
    I'm using XT-CF on my V30-based 086 - perhaps I should look into SCSI there as well? I should have one 8-bit SCSI adapter somewhere.
    Well, any 16-bit IDE adapter will outperform any 8-bit SCSI adapter. Modern CF cards or DOMs can saturate the ISA bandwidth and handle much higher IOPS than any SCSI disk drive ever could at the same time. There is no need to go to SCSI for performance; the only (partial) advantage I see is that SCSI requires its own BIOS and does not suffer from the classical 504 MB size limit on old machines - but neither does XTIDE.

  10. #10

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    IMO you should try the xtal swap anyway. The SCAMP chipset was only rated at 25MHz in 386 applications sure, but that doesn't mean it won't work with a 25MHz 286

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