IBM 5.25 diskette drive adapter - 1.2M drive support

1.2M drives, for the IBM PC family, postdate the IBM 5.25" diskette drive adapter (photo).

The IBM 5.25" diskette drive adapter does not support a 1.2M drive, either for 360K or 1.2M floppies.
The reason is because of data rate.
The IBM 5.25" diskette drive adapter only supports a single data rate of 250 KBPS.
It does not support 300 KBPS nor 500 KBPS, the two data rates used for a 1.2M drive in the IBM PC family.

    = Controller dictates this  (under control of BIOS)
    = Drive dictates this

Note:  The IBM PC family uses fixed-speed drives:  '180K' and '360K' drives always rotate at 300 RPM,  '1.2M' drive always rotates at 360 RPM

  Data rate
(Kbits per second)
per minute
360K floppy in 360K drive 250 300  
360K floppy in 1.2M drive 300 360  Due to a 40-track floppy being in an 80-track drive, the BIOS needs to do 'double-stepping'.
1.2M floppy in 1.2M drive 500 360  


The 1.2M drive was introduced in the IBM 5170.

In the 5170, 1.2M drive support is achieved by the combination of new hardware and software:
• Hardware: The controller supplied in the 5170 supports all three data rates (250/300/500).
• Software: The 5170's motherboard BIOS has code for 'double-stepping', and code for switching the controller between the three data rates.
• Software: Support for 1.2M drives was introduced in IBM DOS version 3.0