5.25 (5 1/4) inch floppy drive 2.4MB ED Extended Density on a PC

Here is another one of those alleged urban legends: the 5.25 inch floppy drive with 2.4MB capacity, twice as much as the regular 1.2MB drive.

These special drives occasionally pop up on eBay. However, since these drives originate from the IBM model 3174 network controller minicomputer, they are not compatible with the x86 PC. Theoretically at least... I decided to prove that theory wrong.

The trick to getting this to work, is to understand how the drive is different from regular 1.2MB 5.25 inch floppy drives.

All floppy drives have a density select pin that can switch a drive between 360KB or 1.2MB (720KB or 1.44MB in case of 3.5 inch drive). The 2.4MB floppy drive behaves differently when triggered on this pin. When set to low density, it does not go into low density mode, rather it slows the spindle speed from 360 rpm to 180 rpm. If data is written at the normal speed of 500kbps, theoretically the tracks are twice as long.

Because the PC BIOS does not recognize this behavior, it is required to cut the density select signal at the motherboard side and short it to ground at the drive side. In that way, the drive will always operate in 180 rpm mode and the BIOS cannot interfere.

Next is the format. Basically the exact same format can be used as with a regular 1.2MB disk, but because the tracks move slower under the head, there is room for more sectors. Therefore the standard number of 15 sectors per track can be bumped up to 30, while using the same sector gap.

Of course, a standard 1.2MB disk does not have the magnetic particle resolution to hold that much data, so over time it will show the same self-erasing behavior of a 1.44MB floppy disk formatted as 2.88MB. But it does work on the short term.

Видео 5.25 (5 1/4) inch floppy drive 2.4MB ED Extended Density on a PC канала FloppyDiskWorkshop