1.5Mb Floppy Drive Support

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

1.5Mb Floppy Drive Support

Postby AltairClone » July 16th, 2020, 9:15 am

I have released an update for the Altair FDC+ firmware. This update adds a new drive type to the FDC+ that supports a 1.5Mb floppy drive under CP/M using high density 5.25” floppy drives like the Teac 55-GFR or 8” DSDD drives.

Many FDC+ owners use high density 5.25” floppy drives with the drive type 4 selection on the FDC+ (5.25” HD floppy as 8” Altair). This duplicates operation of the Altair 8” drive as far as all Altair software can tell and provides the same 308K of formatted storage as the original Altair 8” drive. However, since a number of FDC+ owners use their Altair computer and floppy drives for CP/M more than they use it for Altair BASIC or Altair DOS, I figured it might be useful to get more capacity out of the HD floppy drives than the Altair drive provides. With the new drive type selection, each disk provides about five times the capacity of the Altair drive with a total of 1,525,760 bytes of formatted space. Because the drive and controller appear different to software than the original 8” Altair drive, original Altair software will not work with the new drive, however, I have created a version of CP/M that supports the new drive type.

The 1.5Mb capacity was achieved by:
    1) Using 8” double-density transfer rates with MFM encoding and write pre-compensation instead of single density FM.
    2) Creating a firmware based MFM data decoder to duplicate the hardware PLO and data separator typically required to decode MFM data streams.
    3) Using both sides of the disk.
    4) Using one sector per track to eliminate wasted gap space.
The Altair computer has a hard time keeping up with even the original single density transfer rate, so changes had to be made in the way data is exchanged between the FDC+ and the CPU so that the Altair can keep up at twice that speed for the double density data rate.

Because the disk layout and interchange of data between the CPU and the FDC+ are different for the 1.5Mb drive than for a normal Altair drive, the standard disk boot loader PROM cannot read the boot loader from track 0 of the 1.5Mb disk. To avoid having to install an additional boot loader PROM for the 1.5Mb drives, the FDC+ looks for Altair-style access from the CPU, and if detected, returns a standard Altair sector from FDC+ memory that contains the code required to boot the 1.5Mb drive. The Altair boot PROM thinks it just loaded sector zero, track zero from an Altair disk, so the PROM jumps to the code it just “read from disk” and the boot process continues.

For more information, wee the file "Release Notes v1.3" file in the release .zip file here: https://deramp.com/downloads/altair/har ... are1.3.zip

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