Virtual Sector Generator for 5.25" Hard-Sector Floppy Controllers

VSG Front
VSG Rear
VSG System

The Virtual Sector Generator (VSG) sits between a 5.25" hard-sectored floppy controller and original drives to allow transparent and interchangeable use of soft or hard sectored media. The board is jumper selectable to work with 10 sector or 16 sector controllers. The board is powered by a standard 9 volt battery and automatically powers itself on and off with the host computer. A battery will last a year or more of typical use.

Since 16 sector media – and especially 10 sector media – is getting difficult to find, this is a great way to breathe new life into systems using controllers from North Star, Heathkit, Micropolis, Vector Graphic, and MITS/Altair.

The VSG passes all signals between the controller and the drives except for index/sector pulses. Pulses from the drives are received by the VSG and the VSG generates the index/sector pulses that are seen by the controller. This arrangement is required to handle the specific timing requirements expected by the controllers and software during the first revolution after a drive is selected.

For each of the three standard drive selects, the VSG maintains a running average of the last four revolution times so that the timing of virtual sector pulses matches each drive's rotational rate.

Additional Resources

Limitations

Belt driven drives can have significant variation in rotation time on consecutive revolutions (Instantaneous Speed Variation, ISV). Even though the VSG syncs to the average speed of the drive, a given revolution will vary from nominal by an amount that cannot be predicted by the VSG. If the ISV exceeds 1ms or so, drive operation will not be reliable with soft sectored media.

Almost all of the belt driven drives I have tested work well with the VSG (several each of Shugart SA-400, SA-460, Tandon TM-100-x, Micropolis 1015-x, Pertec FD-200). Drives that don't work reliably with the VSG cannot be linked to a particular brand or model of drive. Instead, if a given drive has acceptable ISV, it will tend to stay that way and work well with soft sectored media. Similarly, if a given drive has excessive ISV, it will tend to remain problematic with soft sectored media and the VSG.

Direct drive motors, typically found in half-height 5.25" drives, tend to be very stable, and therefore, work very well with the VSG and soft sectored media. Fully compatible 48 tpi 360K drives like the Teac 55BV series are still readily available on eBay. Some 96 tpi drives like the Teac 55-GFR can be strapped to work with the older hard sector controllers (e.g., 300 rpm vs 360 rpm, SD/DD versus HD). The media is written at 96 tpi and won't work in a 48 tpi drive, but the media can exchange between 96 tpi drives.

Ordering Information

The Virtual Sector Generator is available as an easy to assemble, through-hole kit, or fully assembled and tested.

Virtual Sector Generator
(Kit)
$40
Virtual Sector Generator
(Assembled)
$60


International customers: The amount charged for shipping during checkout is for US shipments only. An additional shipping charge must be paid for shipments outside of the continental US. After the order is received, an email will be sent with the additional amount that should be submitted.