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Why I used DRDOS 3.41 on IBM PC 5150 (adventures with 256KB RAM, XT-IDE and HXC)

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    I found a minuszero page which states the IBM PC's 63 watt power supply couldn't handle even a single hard drive of the time, but that you might be able to get a Seagate ST-225 to work.

    There's another minuszero page that says the 10 MB drive in the IBM XT was the Seagate ST-412 which was rated by Seagate to have a peak pull of 42W (@12V) + 5.5W (@5V) for a total of 47.5 watts max. You might think that would work with the IBM PC's "63 Watt" power supply, but it wouldn't because each line had a separate rating. The 12V power line in the original PC power supply could only provide 24 watts max, so even one ST-412 would have been too much.

    Those minuszero pages give (nearly) all the details you need to answer what wattage would be necessary for 4x floppies and 2x hard drives:
    Peak watts
    +5 V line
    Peak watts
    +12 V line
    Seagate ST-412
    (10 MB hard drive)
    5.5 W 42 W 2
    (MFM controller card)
    ? ? 2
    Tandon TM100-2
    (Full Height Floppy Drive)
    1.9 W 10.8 W 4
    IBM 5" Diskette Drive Adapter
    (360 KB Floppy Controller)
    2 W 0 W 2

    Multiplying that out gives a peak power draw of 22.6 W on the +5 V rail and 127.2 W on +12 V. The 5V usage is not a problem as the original IBM PC's power supply could provide 35 W. However, for 12 V, not only would the original IBM PC's power supply have failed (24 W max), even the beefier XT power supply, which could provide 50.4 watts on the +12 V line, would not have been sufficient.

    Take my calculations with a grain of salt. As a sanity check, note that the IBM 5161 did power two hard drives with only an XT power supply. So why do my calculations say it could not? (242 = 84W @12V). Partially it is because I'm looking only at the peak wattage, which happens for the hard drives only when first spinning up. If you can time them so that they don't spin up simultaneously (and I seem to recall that devices of that era could do so), you could use the hard drive's typical watts, 20.4W (@12V), for one of the drives. That'd give a max wattage on the +12V line for two ST-412 hard drives of only 62.4W instead of 84W. If I use the amps measured by, rather than the rated amps for the ST-412, the number falls to 52W (@12V).

    That gets us into the ballpark of what the XT power supply was rated to handle, 50.4 W on the +12V line. Still, that seems too close for comfort and not something IBM would have done, so perhaps I am mistaken about the 5161 using two ST-412s. Could it have been a different drive? Are some of the numbers on wrong? Am I just confused about how math works? I do not know.

    Another grain of salt: I'm not sure, but I believe each floppy controller card could only spin up one of its two floppy drives at a time, so there is no physical way all four floppy drives would be using their peak consumption simultaneously. If that's right, at most it would be two at peak (10.8 W at +12 V) and two at normal usage (4 W at +12 V), for a total draw of 29.6 W on the +12 V line, instead of 43.2 W.


      You won’t fit 4 full height floppy drives and two full height hard drives in a 5150, not even with a 5161 hanging off of it. I guess if maybe two of the floppy drives were I some sort of enclosure off the 37 pin port on the floppy controller, but then I’d think that enclosure would have its own power.


        Yea I was thinking of the 37-pin port. I'm planning to try seeing if I can get an HxC disk emulator working off of that port (which can emulate up to two disk drives). In a "real" setup (early 80s) it would have to be a drive enclosure - which good point, true it would probably have its own power. [ and other good point mentioned was that only 2 disk drives at a time would ever be operating ]

        The HxC will need power also - I won't have the internal disk drives connected (hopefully), so I'm planning to just borrow the Molex connector off of one of those (running an extension through the little hole at the back of the case).

        Also good reminder that the original 5150 (by itself) wasn't even intended for 1 hard drive, let alone 2. Well, I'm not ashamed of "cheating" with an XT-IDE. The HxC's are very nice (for getting software images from a modern PC and running them on a 5150), but I don't want to mess with the drive bays to try to fit a 3.5" device. The XT-IDE just lets me keep all that file content in one place, instead of swapping between 180/360/720KB images.
        Last edited by voidstar78; August 28, 2021, 02:00 PM.
        IBM 5110 ['78], PET 4016 ['80], C64 ['82], IBM PC 5150B ['84]. A retired SysOp, creator of, VUC, ANT


          I got the 37-pin port working just fine. Not the most efficient way, but I put some notes here:

          5150: Setting up Floppy Disk Controller – voidstar

          BTW, anyone know why DR DOS 3.41 doesn't have enough memory to fully run CHKDSK ?

          My config.sys and autoexec.bat are empty (blank, renamed them -- such that on boot up, the OS asks for time and date). So it's a completely "default" boot up of DR DOS 3.41

          I have the full 640K RAM enabled.

          I can do chkdsk /a, just not the more thorough chkdsk.

          IBM 5110 ['78], PET 4016 ['80], C64 ['82], IBM PC 5150B ['84]. A retired SysOp, creator of, VUC, ANT


            Try reducing free memory to less than 512K. Having a signed memory check would overflow if the free memory exceeds 512K.


              I was wrong. I set up a 640k virtual machine with 46MB hard disk and chkdsk worked properly. I should have realized before posting that DR-DOS from 1988 would work with 640K and few utilities loaded.


                Oh, the size of the partition might be too large? It's DR DOS 3.41, its installer created the partition, and a ton of other apps are running (lotus, Keen4, mach3, procomm plus). But true, the OS might work, but chkdsk could be failing on a 128MB partition. It has 16MB left at the moment. Or some DRDOS/XTIDE/CF incompatibility, who knows.

                I did try EATMEM... tried at 500K, 400K, 300K, 200K, 100K... all same "not enough memory" reported by chkdsk.

                Like maybe it tries to allocate some buffers proportional to the size of the partition, and that's what it doesn't have enough memory for.

                Bummer, too lazy to try to split the partition. I guess I could try like "CHKDSK A:", except I already took out the FDC and bolted back up the case.

                Well, is there a pre-1990 RAMDRIVE? Yeah, like a version of EATMEM, but instead of just eating the RAM - make a drive letter out of it

                IBM 5110 ['78], PET 4016 ['80], C64 ['82], IBM PC 5150B ['84]. A retired SysOp, creator of, VUC, ANT