André's 8-bit Pages  Projects  Code  Resources  Tools  Forum
(by Google)

PET index - Disk drives

This part is under construction

The Commodore disk drives for the PET computers actually are more intelligent than the PET itself - they had two 650x processors, one for the DOS and one for the drive controller. Using this "intelligent" design the PET was able to send a command to the disk drive and continue its own work, while the floppy was doing its.

Disk drives to be used with the PET have to be equipped with an IEEE488 interface. Most of the devices had two drives, but there are single drive units.

In the following you find some more information about these 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

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.

ERROR INDICATIONS
NUMBER OF FLASHESERROR CAUSECOMPONENTLOCATION
1 Zero Page 6532 UC1,UE1
2 ROM $E000-$EFFF 2364 UL1
3 ROM $C000-$DFFF 2364 UH1
4 Unused
5 Zero Page 6530 6502 UK3 UH3
6 Common RAM $1000-$13FF 2114 UC4,UC5
7 Common RAM $2000-$23FF 2114 UD4,UD5
8 Common RAM $3000-$33FF 2114 UE4,UE5
9 Common RAM $4000-$43FF 2114 UF4,UF5
10 Controller ROM 6530 6502 UK3 UH3

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.

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.

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.


Return to Homepage

Last modified: 2018-12-29
follow

Follow my 8-bit tweets on Mastodon (In new window) or Bluesky

discuss

Discuss my site on this 6502.org forum thread

(Forum registration required to post)

hot!

Dive into the retro feeling and build yourself a Micro-PET or a Multi-board Commodore 4032 replica

Need more speed? Speed up your 6502 computer with this 10 MHz 6502 CPU accelerator board

Interested in electronics design? Look at the design lesson I got from Bil Herd, the hardware designer of the C128

Want 64bit? - pimp the 6502 with the 65k processor design!