FDD Interface and Cabling

The FDD interface and power requirements are completely standardized, as follows:


PC-class systems used a separate FDD controller card. XT- and AT-class systems and some early 386s used a combination HDD/FDD controller card. Current systems use an embedded FDD controller. These controllers differ only in their maximum data rate, which determines the FDD types they support. Early controllers run at 250 Kb/s, which supports only 360 KB 5.25” FDDs and 720 KB 3.5” FDDs. Later controllers run at 500 Kb/s, which supports any standard FDD, or at 1 Mb/s, which is required for 2.88 MB 3.5” FDDs. Run BIOS Setup to determine which FDD types a given system supports.


If you must install a higher capacity FDD than the controller supports—e.g., if you must salvage data from a hard drive in a system whose old 5.25” 360 KB FDD has failed and you have only a 3.5” HD FDD to replace it—you have two alternatives:

  • Remove or disable the on-board FDD controller, and replace it with a third-party FDD controller that supports the higher capacity FDD.

  • Lie to the old system about what type of FDD you are installing. For example, install a 1.44 MB FDD, but tell the system that it is a 360 KB or 720 KB FDD. All FDDs run at 300 RPM, except 5.25” 1.2 MB FDDs, which run at 360 RPM. That means that any ED, HD, or DD 3.5” FDD can emulate any lower capacity 3.5” drive, as well as the 5.25” 360 KB FDD. Use blank DD diskettes, and format them in the new drive. Copy data to the floppies ...

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