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Thread: Shugart 850 rebuild

  1. #1
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    Default Shugart 850 rebuild

    After years of curiosity I finally obtained an 8 inch FDD, a Shugart 850. Thanks to Al Kossow and Bitsavers I also was able obtain the Shugart service manual. The drive I acquired is very dirty, has seen some mishandling and is in unknown operating condition. (Somebody cut the wires rather than unplug it, for example.) I am in the process of attempting to rehabilitate it.

    I have stripped it down as far as I think is prudent. I know that I don't want to disassemble the track 0 sensing or head carriage positioning mechanisms. But there is a head-loading mechanism that might be safe to remove, and the front latch solenoid doesn't appear to be critical. The disk-eject mechanism and write-protect switch are pretty simple, too.

    But I'm wondering if I can disassemble the spindle clamp or remove the spindle sensors, which are a dual-sensor setup. There appears to be some leeway in their positioning and I don't know how critical their alignment is, either relative to the heads or sender to receiver. At the moment they appear to me to be slightly staggered, top to bottom.

    If anyone with knowledge of the topic has any pointers, I'm all ears. Thanks.

    -CH-

  2. #2
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    Curiosity overcame caution and I stripped the drive to the casting for a thorough cleaning. I can rebuild it again - thanks to the service manual - but I will need to make several tests and adjustments after doing so. The final tests will involve alignment: it would be best if I had an 8-inch AAD but next best, according to Chuck(G) et al. is a manufacturer-issued disk which presumably conforms to standards for interchangeability.

    Thinking about where to find such a disk has raised another question: Although I have never seen one I know the early standard for floppy-disks was "hard-sectored" and that "soft-sectoring" was a later simplification (in a sense). I believe the difference between the two was the number of "index" holes punched into the floppy disk itself. A hard-sectored scheme had a multitude of holes and assigned one sector per hole. This would imply that the sizing of the sector was limited compared to a soft-sector scheme which could assign a variable number of sectors of variable sizes, all identified by their relative offset to the one index hole on the floppy. Drive electronics, size of the disk and rotational speed would influence the theoretical limit of the size of a sector, but the point is that soft-sectoring was more flexible. At least that's the way I understand it.

    So this Shugart 850 has a companion drive, the 851, which is reportedly also DSDD like the 850, but has a jumper-selectable option of reading either single-index or multi-index floppy disks - hard or soft-sectored, in other words. I don't believe this is an option on the 850. But this is where the questions arise: Is there more than one standard or convention for the number of index holes in a hard-sectored floppy disk? I have seen numbers suggesting 32 and 26, though I don't know if either is correct. And can a soft-sectored drive read a hard-sectored floppy, or will the flurry of index pulses throw everything off?

    As a practical matter, am I going to have to find a manufacturer's distribution 8-inch soft-sectored DSDD disk to perform my alignment?

    -CH-

  3. #3
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    If you're not intent on writing floppies that will be used by others, a good, known floppy with data may be all that you need. If you're hooking one to a PC, just use ImageDisk's alignment function.

    As regards the door latch solenoid, I remove them--they're more trouble than they're worth--and they have nothing to offer with most common software.

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    Quote Originally Posted by Chuck(G) View Post
    If you're not intent on writing floppies that will be used by others, a good, known floppy with data may be all that you need.
    No, software distribution is not the goal. I have ten IBM disks, 8-inch, soft sectored, some IBM issued (see example).

    Quote Originally Posted by Chuck(G) View Post
    If you're hooking one to a PC, just use ImageDisk's alignment function.
    Understood. Connection to a PC was the original impetus. Additionally, I have no other exerciser.

    Quote Originally Posted by Chuck(G) View Post
    As regards the door latch solenoid, I remove them--they're more trouble than they're worth--and they have nothing to offer with most common software.
    Door latch is the least of my worries. If I put this back together and everything works there will be no one more surprised than I.

    But thanks for your reply.

    Now, just to muddy the water a little more: back in 2009 http://www.vcfed.org/forum/archive/i...p/t-13849.html you and Lorne and MikeS and Dwight engaged in the discussion of soft-vs-hard sectoring and you stated then that SOME controllers can adapt to the extra pulses generated by a hard-sectored disk. Not mine, from what I have read. So if I acquire any more 8-inch floppy disks I will need to be sure they are soft-sectored / one hole. I should not assume the 850 controller can handle anything more than 1 index hole. Correct?

    -CH-

    Disk.jpg

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    No, some drives can--they separate the sector hole signals from the index hole signal and route them to a different output pin. You could additionally select how the 32 sector holes were to be realized at the output; as 32, 16, 8 or 4 sectors.

    This, of course, doesn't mean you can use a regular (765-based) disk controller to read hard-sectored disks as the convention was pretty much sui generis on encoding schemes for hard-sectored media (no standards). I usually use my Catweasel on these and puzzle the result out from there.

    Having said that, I should observe that in some cases, it was easier to come by hard-sectored media than soft, particularly where the market catered to early word processors. So, by setting the drive up to separate sector signals from the index, you could use the drive with both hard- and soft-sectored floppies as soft-sectored.

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    Quote Originally Posted by Chuck(G) View Post
    No, some drives can--they separate the sector hole signals from the index hole signal and route them to a different output pin. You could additionally select how the 32 sector holes were to be realized at the output; as 32, 16, 8 or 4 sectors.
    Were the index holes (of a hard-sectored disk) on the same radius from the center of the spindle as the index hole, or not? This 850 has two pairs of index sensors, about 5 degrees apart and I thought maybe offset as well. I didn't know if it was for redundancy or another purpose.

    Quote Originally Posted by Chuck(G) View Post
    This, of course, doesn't mean you can use a regular (765-based) disk controller to read hard-sectored disks as the convention was pretty much sui generis on encoding schemes for hard-sectored media (no standards). I usually use my Catweasel on these and puzzle the result out from there.
    I thought the Catweasel could only be found in childrens' books; I've never seen one for sale. What happened with the PIC device you built to monitor index pulses?

    Quote Originally Posted by Chuck(G) View Post
    Having said that, I should observe that in some cases, it was easier to come by hard-sectored media than soft, particularly where the market catered to early word processors. So, by setting the drive up to separate sector signals from the index, you could use the drive with both hard- and soft-sectored floppies as soft-sectored.
    NOS on eBay is all hard-sectored, at least for now.

    But at the moment the cart is way out in front of the horse and I needn't be worrying about sector holes until I see if the spindle will move. Thank you for your reply.

    -CH-

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    Yup on the sector hole spacing. All (but for some oddball Memorex very vintage stuff) HS 8" are 32 sector. So the drive uses a couple of one-shots to differentiate the timing between sectors and the index. Very simple circuit, really.

    I've still got the little PIC device somewhere in my collection of old stuff.

    As far as Catweasel, I'm slowly moving away from that and migrating to my STM32F407 MCU version. Same functionality, just not dependent on a PC. (with a half-meg of program space, you've got a lot of room to do interesting things). I already use a 407 to handle 7-track 1/2" tape and am plodding along on a Pertec 9-track interface. Arduinos are for amateurs.

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    Quote Originally Posted by clh333 View Post
    Were the index holes (of a hard-sectored disk) on the same radius from the center of the spindle as the index hole, or not? This 850 has two pairs of index sensors, about 5 degrees apart and I thought maybe offset as well. I didn't know if it was for redundancy or another purpose.
    Radius is the same; the different offset is for single- and double-sided disks, the hole closer to 12 o'clock is for single density.
    See post #13:
    http://www.vcfed.org/forum/showthrea...t-drives/page2

  9. #9
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    Quote Originally Posted by MikeS View Post
    Radius is the same; the different offset is for single- and double-sided disks, the hole closer to 12 o'clock is for single density.
    See post #13:
    http://www.vcfed.org/forum/showthrea...t-drives/page2
    Thanks, that explains why there are two sensors and what that leading-edge notch is for.

    -CH-

  10. #10
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    Quote Originally Posted by Chuck(G) View Post
    ... I'm slowly moving away from that and migrating to my STM32F407 MCU version. Same functionality, just not dependent on a PC. (with a half-meg of program space, you've got a lot of room to do interesting things). I already use a 407 to handle 7-track 1/2" tape and am plodding along on a Pertec 9-track interface. Arduinos are for amateurs.
    And here I thought I was au courant just knowing what an Arduino was. I'm not even gonna ask how the STM32F407 is implemented.

    But thanks for your reply.

    -CH-

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