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The Commodore PET (Model : CBM 8096)



As might be expected for a computer with such a large range of models, there were also a large range of options available, including Commodore's own disk drives and printers, as well as third party add-ons. I shall only be documenting the options that I have, or am very interested in obtaining - however unlikely that may be.


Disk Drives

Commodore produced a large range of floppy disk options which used a number of different formats. This page only describes the 8250 model, which features 2 x full height, 5.25, Double Sided / Quad Density drives.

Although the disk drives were Quad Density, they were not Dual Density, and could not, for example, read the lower density Commodore 1541 disk format used on the Commodore 64.

Model 8250

Disk Drives 2 x 5.25" DS/QD 300 RPM
Tracks 77 / side
Tracks / inch (tpi) 100
Sectors/Track 23-29 (2083 / side)
Bytes/Sector 256
Capacity 1.04MB (520KB / side)
Coding Group Code Recording (GCR)
Access time 5ms
Track/Track seek 125ms
Interface IEEE-488
Firmware 16kB ROM with DOS 2.7
Disk buffer 4kB RAM
Dimensions (h x w x d) 180mm x 380mm x 395mm
Weight 12.6 kg
The disk drives interface with the PET using the IEEE-488 interface.

Photo from the Commodore Series 2000 User Guide, showing the rear of a disk drive case and two IEEE-488 cables - a standard "piggy-back" cable and the PET edge connector to IEEE cable.

The piggy-back cable is a standard 1m IEEE-488 cable, these are quite common. The PET edge connector cable was a "special" designed by Commodore for the PET IEEE interface.

The Commodore cable is hard to find these days, but I bought an adapter for use with a standard IEEE-488 cable (bought for 5.50 from ebay) from RETRO Innovations in the US for $10 + shipping.

The disk controller is installed under the cover and a single interface board for the two disks is installed on the right hand drive.

Commodore built their own drive products, the 8050 and 8250 used 100 TPI drives manufactured by Tandon, MPI and Micropolis (2 models). Commodore removed the OEM's electronics and replaced them with their own.

As you can see in the photo, the 34 way ribbon cable usually seen on floppy disk drive interfaces is not present - after modification by Commodore, these drives were no longer Shugart compatible.


Credits :

Picture from Dave Dunfield's Classic Computer site's PET pages

Technical data reference Herb Johnson

A close up of the disk controller board, which includes 2 x 6502 processors (1MHz), 16K ROM, 4K RAM - the drive controller is computer in its own right and included the Disk Operating System (DOS Version 2.7) on the board.


Credits :

Picture downloaded from Commodore Computer Online Museum    

A close up of the disk controller board, with the major components identified from the Dual Drive Service Manual

Data was transferred from the 8250 to the PET over the IEEE-488 bus at 1.8 KB/s. (The later VIC-20 and Commodore 64 used a serial version of the IEEE bus and only managed a throughput of 400 B/s)


Madison Computer Z-RAM
Madison Computer developed Z-RAM, a Z80 based add-on card that enabled the PET to run CP/M.

The 6502 was removed from the PET main board, the Z-RAM card incorporated a replacement 6502, a Z80A CPU  clocked at 4MHz and 64KB of RAM. To the "new" 6502, the Z80 appeared as a number of additional I/O ports.

It would be really great to have a Z-RAM for my 8096, but the chance of finding one would seem to be exceedingly slim.

Credits :

Details and image courtesy of Mike Naberezny





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