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TAC Newsletter Topics

  • Printing from a IIgs to an Apple LaserWriter
  • Using 3.5" drives on an Apple IIe, IIc, IIc+, IIgs
  • ImageWriter II Printing Problem
  • Using Classic AppleWorks on Your new iMac
  • Apple IIc Power Supply
  • Requirements for the Macintosh Apple IIe Card
  • Apple IIgs Battery Replacement
  • Diskettes With Bad Blocks
  • Connecting an Apple IIgs to a Mac
  • Image Writer II Network Printing
  • About 3.5" drives for the Apple IIc
  • What is Interleave?
  • ProDos volumes and mapping

    Using 3.5" drives on an Apple IIe, IIc, IIc+, IIgs By Ridge Prevost, Tucson Apple Core

    Apple shipped several versions of external 3.5" drives: the beige single-sided 400k original Lisa/Mac drive; the beige Mac-only 800k drive; the white IIe/c-only (IIgs, too, but slow data rate) 800k UniDisk 3.5 (with onboard 65C02 chip); the platinum 800k Apple 3.5 for use on Macs and Apple IIgs & IIc+; the 1440k platinum Apple FDHD SuperDrive for use on FDHD Macs and IIgs/IIe's with the SuperDrive controller card.

    The Apple external SuperDrive (FDHD) also has the capability of being used as an 800k drive, and normally requires the Apple "SuperDrive" controller card to enable reliable data transfer on a "slow" Apple IIe. As I understand, it works in 800k mode with a UDC (Universal Drive Controller card from a 3rd party.)

    The Apple FDHD drive and card are optimized to transfer data to/from a diskette initialized (formatted) at an interleave of 2:1 (every other block on a track is sequential) except on a IIe. The IIe's CPU is so slow that the optimal interleave for 3.5 diskettes initialized on it is 4:1 (every fourth block on the track is sequential).

    When one has an Apple UniDisk 3.5 (white color) and its controller card attatched to a IIe/c and uses the Apple IIe/c System Utilities to initialize (format) a 3.5" 800k diskette (Copy II Plus, also), it sets the interleave to 4:1. This UniDisk 3.5 has a 65C02 microprocessor on board to handle the slow data transfer rate between the drive/card and the IIe/c's system bus. If you then use a diskette initialized at 2:1 (on a Mac with the ProDOS extension or a IIgs) on this setup, the data transfer rate is extremely slow as the diskette has to spin twice around to get to the next sequential block.

    The Apple IIgs and Mac have fast enough CPUs so that they can read data on diskettes that are formatted at a 2:1 interleave. That means that they can assimilate every other block as the diskette rotates at its normal speed. The IIe/c have slow CPUs, so they need to work with diskettes formatted at 4:1 interleave; they can only assimilate every fourth block of data as the diskette spins at its normal speed; if a 2:1 diskette is used, the next block of data is not on the fourth block, as expected, but all the way around again; this makes for very slow data transfer.

    In general, with the Apple IIe, one should stick with these combinations: white UniDisk 3.5 and its own controller card; platinum 800k Apple 3.5 (or clone 3.5 drive) and a UDC card; platinum Apple FDHD SuperDive and its own controller card.

    With the original Apple IIc that has the ROM chip upgrade, the only drive that will work is the white Unidisk 3.5. With the Apple IIc Plus, you can use either the UniDisk 3.5 or the platinum 800k Apple 3.5. With the Apple IIgs, you can plug into the built-in disk port the white UniDisk 3.5 (expect slow transfer rates) and the platinum 800k Apple 3.5 drive; to use the Apple FDHD superdrive in 1440k mode, you need to connect it to its own controller card and set that slot to "Your Card".