PET index - Disk drives

•  
  2018-12-29 Added link to "REL" files on the 2040
•  
  2017-10-15 Updated links on the Hard Drive page.
•  
  2017-09-29 Added hardware information.
•  
  2010-01-04 Started this page

• Quickfinder
• Capacity
• Disk drive error codes
• Drive hardware information
• • 170k Drives
  • 8250/1001 Low Profile Drives
  • ROM differences related to the drive
• Other disk drive information

• Disk drive list
• Architecture overview on the drives and schematics
• DOS ROM information
• Drive utility programs
• Gallery with pictures of the drives
• Information about the Commodore harddisks
• Drive capacity and compatibility and other DOS information.
• Disk error codes
• Other disk drive information

All in all, there were two major 5.25" disk formats for the PET series, the 35 track double density version with around 170 kByte and the 39/78 track quad density version with 512k/1M Byte free. Both major types are completely incompatible.

The quad density came in two variations only, single sided (8050 DOS2.5/2.7) and double sided (8250/1001 DOS2.7). The double sided drives could read the single sided disks ok. The single sided drives could even read the double sided disks, as long as the second side was not used.

The double density disks are another story. With DOS1 Commodore introduced a disk format that is almost the same as in the 1541 - only in one of the speed zones there was one sector more. Therefore the disks had 670 blocks free. Probably Commdore found that there were too many errors, so with DOS2 they reduced the number of blocks by one sector per track in one speed zone, resulting in 664 blocks free. But that is not the end of the story. In the disk format there is a write gap between the header and the data block. Commodore changed the size of the write gap from the DOS2 disks to the 1541. Due to this the drives are only read compatible. You can read more about this here.

The Commodore disk drives indicated hardware failures detected during boot with a blink code. Here's the meaning from the 8050/8250 service manual:

POWER UP DIAGNOSTIC FOR 8050/8250 FLASH CODE

On power up, the three LED's on the drive should blink. The drive LED's go off and the center Tri-State LED remains green. The sytem has a built in self diagnostic which can be read in the form of a flash code. In the event of a failure, the 3 LED's will blink in a repetitive sequence. The number of blinks between intervals will indicate the possible location of a failure.

If the LED's come on and stay on, the microprocessor UN1 is probably at fault.

The disk drives used in the PET drives are now rather old. Some if not most of them are failing.

170k Drives

The 170k drives actually used a stripped down Shugart SA-400 drive. It was lacking the drive electronics and was thus named SA-390

This drive is in fact used in the Apple II disk drives as well

8250/1001 Low Profile Drives

The large double sided 80 track drives used a Panasonic JU-570 mechanism.

They come in two variants, the JU-570 and JU-570-2 drives. The shorter JU-570-2 drives can be used in all drives, the longer JU-570 only in the SFD-1001, but not in the 8250-LP or 8296-D.

Unfortunately these drives suffer from leaking electrolytic capacitors. Please see the link for a description how to repair them.

• JU-570-2 repair Website is in German though

ROM differences related to the drive

The disk drives themselves were procured from different providers, so the ROM had to be adapted to the different hardware versions as well. In the PET disk drives this was stored in the RRIOT chip used by the disk controller CPU (remember, the drives had two processors!).

Please see the link for a list of disk drive types and matching ROMs.

• Drive types and matching ROMs Website is in German though

The file drive_info.txt contains a wealth of drive information.

8050/8250 Service Manual on the CBM archive, or in HTML on a Finnish server.

Some tools for the Commodore PET Drives, esp. the 1001/8250

More information on other, non-CBM harddisks on the Commodore Harddisk complete history page, with links to many more details on the CBM 9060 and 9090 drives as well.

Further information on drives on portcommodore.

• Using direct files with the 2040 (without REL files) Using a SEQ file as index of records in another data file (from Compute! issue 1)