1980s Vintage Computers

Hewlett-Packard Series 80 - HP-9121 disc drive


The HP-9121 is an Amigo protocol drive that is ideally suited to the Series 80. It uses 3.5" floppy disks. These have to be double density (720kB) rather than high density (HD - 1.4MB), as that was the highest capacity available when the drives were made, and using HD disks can lead to data being lost and hence disk errors.

As the unit uses single sided drives, and doesn't use the maximum number of tracks and sectors available, the actual capacity per floppy is 270kB. HP also made this drive 100% backwards compatible with the 82901M 5.25" drive, so the two are fully interchangeable on any Series 80 machine.

The drive contains a 6809 processor with an 8K ROM and 2K RAM. The ROM firmware has been reverse engineered by Holger Veit, this source code has been heavily commented and provides a detailed understanding of the drives internal operation and handling of the Amigo protocol.

For manuals please see the HP Computer Museum page.

HP Series 80 tips


This page was last revised on: 12/04/09