The Commodore 8250 disk drive was the highest capacity floppy drive Commodore made for its 8-bit computers. It was a dual drive unit the connected to the IEEE-488 bus used on the PET and CBM line of computers. Each drive stored one megabyte of data on 5.25 inch double sided quad density disks. When you listed the directory on a Commodore equipped with an 8250, the disk read 4133 blocks free.
Commodore CBM 8250 disk drive
The original Commodore CBM 8250 disk drive, originally released in 1982, looks very much like other Commodore dual drive units such as the 4040. It used full height drives made by Micropolis, Tandon, or MPI. Even though it looked almost exactly like other Commodore dual drive units, the media was not forward or backward compatible with other Commodore dual drives except the single-sided, quad-density 8050 drive.
The quad density disk format didn’t last very long. Two other notable computers that used it were the Digital Equipment Corporation Rainbow, and the Tandy 2000, a pair of quirky early MS-DOS machines that weren’t fully IBM compatible.
Since Tandy found a way to read and write double-density disks on its quad-density drive, theoretically Commodore could have done the same and made the 8250 compatible with the 4040 and 1541, but they didn’t.
By cutting one or more of three jumpers at location UE1, or lifting pins 22, 23, or 24 of the 6532 at UE1, you could assign an 8250 a drive number between 8 and 15. To change to device 9, you would cut the jumper next to pin 24.
The IEEE-488 bus was also used on HP computers. Commodore did not use it on its popular home computers because of the cost. The IEC bus on the Commodore 64, 128, VIC-20, and 264 line was derived from the IEEE-488 bus but not compatible with it. To use a Commodore 8250 with a Commodore 64 or 128, you have to get a third party IEEE-488 adapter that plugs into the cartridge port.
Commodore 8250 LP
The 8250 LP is a cost reduced version of the 8250, released in 1983, that used half-height drive mechanisms. This made it considerably smaller than the original 8250. Commodore’s 8250LP used Panasonic drive mechanisms, the same as the SFD-1001. The styling was lower and swoopier, in keeping with the B-128 and the rest of the so-called “Porsche PET” line.
The success of the C-64 led Commodore to abandoning this line because they didn’t have enough production capacity to keep up with the C-64 and the Porsche PETs. Commodore may have produced the enigmatic SFD-1001 as a way to quickly dispose of its remaining 8250LP drive mechanisms.
Neither variant of the 8250 is super common. They came late in the PET’s service life, and Commodore didn’t promote them heavily. But Commodore BBS owners did covet the 8250s into the late 1980s and beyond. They do occasionally turn up on Ebay.
Quad density disks were never especially common and high density disks quickly overshadowed them. You can format regular double density disks as quad density disks in a Commodore 8250 disk drive and it usually works. I don’t recommend it, but if you can’t find quad density disks, this trick allows you to use the drive.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.