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 main 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.


Note that the IBM PC family uses fixed-speed drives:  '120K/180K/320K/360K' drives always rotate at 300 RPM,  a '1.2M' drive always rotates at 360 RPM

  [A]

Data rate
(Kbits per second)		 [B]

Revolutions
per minute		 [C]

Revolutions
per second		 [D]

Kbits per track
(per revolution)

  Comments
360K floppy in '360K' drive 250 300 5 50  
360K floppy in '1.2M' drive 300 360 6 50  BIOS needs to do double-stepping.  See note 1 below.
1.2M floppy in '1.2M' drive 500 360 6 83.3r  
      [B] ÷ 60 [A] ÷ [C]  


Regarding the [D] column above: That is unformatted capacity.

A 360K floppy has 50 Kbits per track.  You can see that when a 360K floppy is in a '1.2M' drive, in order to achieve 50 Kbits per track, a data rate of 300 Kbits per second is used to compensate for the 1.2M drive's faster rotational speed of 360 RPM.




BY THE WAY

The '1.2M' drive was introduced in the IBM 5170.

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






Note 1 Double-stepping is required to be done by the BIOS, due to a 40-tracks-per-side floppy being in an 80-tracks-per-side drive.
That BIOS could be the motherboard BIOS, or for a 'smart' floppy controller, the controller's expansion BIOS.
Some motherboard BIOS' predate the '1.2M' drive, and therefore, do not know about double-stepping.