PET index - disk drives

There a a great variety of 5.25" disk drives. For the first PET with BASIC 2 it started with the 2040 disk drive and with its CBM DOS 1.0. The design of this drive is typical for all the other dual drive disks up to the 8250, and even for the single drive 1001.

Those drives have two 6502 processors (i.e. one 6502 and one 6504). The first processor (the 6504) connects to the drives and does all the disk read/write operations. It uses a hardware(!) GCR decoder to convert the data bytes to the GCR encoding that is used on the disk (for more details, see the Architecture overview). The second processor (the 6502) does the IEEE488 transfer to the PET and all the file and directory handling. Both CPUs have their separate ROM, and some separate RAM. They communicate via a 4k common RAM area. The DOS part - the 6502 ROM - was the same for a product line. The FDC part - the 6504 ROM - however can be different for different drive mechanics.

The 2040 is a dual drive floppy disk. Its DOS 1.0 has a disk format that is very similar to the 1541 disk format. Only in one of the speed zones the format has one additional sector (resulting in 670 blocks free after format). The DOS lacked the relative files, though. It is assumed that the 3040 only is a renaming of the 2040, to accompany the 30xx PET models. It has DOS 1.2, that was also shipped in later 2040 drives.

The 4040 then got DOS 2.0. It features the same format as the 1541 (664 blocks free) and relative files. Later it then got DOS 2.1. These drives were the first to sport relative files.

DOS 2.5 then came with the 8050 drives and DOS 2.7 came with the 8250 drives. Those drives feature 500k on one single-sided disk (8050) or 1M on a double-sided disk (8250)! The new DOS was needed to handle the 77 (8050) resp 154 (8250) tracks resp. The internal drive architecture is still the same as the 2040, with its 2 processors etc.

DOS 2.7 is even capable of being used in the 4040, the 8050 and 8250 without changes. The DOS ROM checks the FDC for the type of drive (170k, 500k or 1M) connected and adjusts its tables appropriately. However, besides some 4040 prototypes this DOS is only seen in 8050 and 8250 drives.

The SFD1001 is a single drive disk drive. Basically it is a single-drive 8250. In fact it uses the same ROM. Only the FDC always returns an error for the second drive.

The 4031 and 2031 seems to be single drive 4040, yet they are not. It they are in fact the sister drive of the 1541. As such they have only one CPU and the two ROM parts (DOS and FDC) have been put together into one. The FDC code has been put into the interrupt routine, while the DOS ROM is the main loop. Also the hardware GCR encode has been removed, so software GCR encoding and decoding routines have been added. The DOS version here is 2.6. They came in high-profile (2031 and 4031) cases as well as 1541-like low-profile cases (2031lp). See for example here

The early 1540/1541 manual says that DOS 2.6 is read/write compatible with the 4040 DOS. However, rumors have it that the write gap between header and block is different between the two, so that they are only read compatible (Read more).

Here is a list of disk drives for the Commodore PET

The "LP" versions were "Low Profile" versions. Which means that while the other drives were delivered in the old "boat anchor" design with full height drives, the LP versions came in 1541-like (SFD-1001) design or even in a very slick low profile case for the 8250LP. You can see them in the drive gallery.

Commodore even sold 8" disk drives...

... and harddisks for the PETs.

Note: the DOS version numbering of "3.0" for harddisks and even later 8" drives seems inconsistent with the DOS 3.0 of the 1571 disk drive for the C64/C128.