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)
Revolutions per minute
360K floppy in 360K drive
360K floppy in 1.2M drive
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
BY THE WAY
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