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RE: disks for Altair drives

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  • mfeberhard
    Bill: Ah. The extra holes in the sleeve are there because the location of the index holes is different for single-sided and double-sided disks. Some disk
    Message 1 of 5 , Sep 23, 2013
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      Ah. The extra holes in the sleeve are there because the location of the index  holes is different for single-sided and double-sided disks. Some disk sleeves were punched to work with both kinds of drives.

      On the other side of the equation, some drives (e.g. the SA850/851) has two sensors - one for single-sided disks, and one for double-sided disks. These drives also have an output that tells the controller whether or not the disk is single- or double-sided. Some of these drives (and  some controllers) get confused if they are getting index pulses on both sensors, and you would need to cover up one set of sleeve holes for things to work correctly.

      The Altair (Pertec) drives have only one index-hole sensor, so the Altair doesn't care at all about the other punched holes. Disks with the extra sleeve holes will work just fine.


      I don't think so with the 88-DCDD. The spec says that the max data bytes per sector is 137, and I think the hardware will simply not let you read or write a longer sector.

      Interestingly, you can write huge sectors with the Processor Technology Helios controller, which also uses hard-sector disks. For the Helios, the minimum sector is 2 sector-pulses long (256 bytes), and can be any multiple of 2 sector pulses in length. When you make large sectors, the space between the sectors gets used for data, giving you about 10% more data. So for example, a sector that's 4 sector pulses long will have more than 512 bytes. PTDOS actually does this, allowing any sector sizes, so long as they all add up correctly on any track.

      --- In altaircomputerclub@yahoogroups.com, <grant@...> wrote:

      Is it possible to use a disk with special formatting that has only one sector per track?  

      On Mon, Sep 23, 2013 at 12:12 PM, <eberhard@...> wrote:

      A hard sectored disk (of the type required by the 88-DCDD controller) has 32 equally spaced sector holes, plus one more index hole that is half-way between two of the sector holes. These are the only type of disks that will work on an Altair.Soft sectored disks have just one index hole. Soft sectored disks work only with a soft sectored controller, typically based on one of the Western Digital disk controller chips such as the WD1773 or the WD1793.A soft sector controller includes logic that can find specially-encoded sector headers. This is how the controller finds a sector.Hard sector controllers have no such logic, and the disk format does not include the specially-encoded sector headers. Rather, they count the sector-hole pulses starting at the index hole to find a sector.There is no way that soft-sector disks (those with just an index hole and no sector holes) will work on an Altair with an 88-DCDD controller.

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