Enhanced Floppy Disk Controller for the Altair 8800

Breathe New Life Into Your Altair

The Altair FDC+ is an enhanced version of the original MITS 8" floppy disk controller for the Altair 8800. The FDC+ is a 100% compatible drop-in replacement for the original two-board Altair FDC. The FDC+ can serve as a replacement for a missing or defective Altair FDC, or you can use the FDC+ as a reference point while restoring original floppy equipment to working condition.

In addition to providing disk controller functions, the FDC+ includes up to 64K of RAM and 8K of PROM that can be separately enabled if needed. This RAM and PROM can come in very handy when trying to get an Altair up and running. The RAM can be enabled in 1K increments. The PROM can be enabled in 256 byte increments.


And There's More...

The FDC+ can be used as a drop-in replacement for the original two board Minidisk controller for the Altair. The FDC+ can connect to a Minidisk drive cabinet or directly to the Altair Minidisk buffer boards in an Altair 8800bt Foley configuration.

The FDC+ can connect directly to readily available Shugart SA-800 (or equivalent) drives and make them function identically to the Altair/Pertec drive and cabinet. From the perspective of the Altair computer and software, it appears that an Altair/Pertec drive is attached. Even the media itself is interchangeable – the Shugart can read floppies written on an Altair/Pertec drive and vice versa. In addition to being more readily available, more reliable, and less expensive than the Altair/Pertec drives, the Shugart drives don't require the additional buffer board present in the Altair drive cabinet. Just supply power and connect the Shugart drives directly to the FDC+ with a ribbon cable.

The FDC+ can connect directly to 5.25" Shugart SA-400 (or equivalent) drives and make them function identically to the Altair Minidisk. In this configuration, the Altair buffer board is not required between the controller and the floppy drive. From the perspective of the Altair computer and software, it appears that an Altair Minidisk cabinet is attached. When connected to 48 TPI drives, the media itself is also interchangeable with an Altair Minidisk. 96 TPI drives can also be used, and the media will interchange between 96 TPI drives, but the 96 TPI written media won't work in a 48 TPI drive and vice-versa.

The original Altair FDC requires hard sectored media for both the 8" (32 sector) and 5.25" (16 sector) formats. However, the FDC+ can also work with soft sectored media. If the controller determines that sector holes are not present as a floppy spins up, it syncs to the rotational rate of the floppy and generates virtual sector pulses so that everything else "just works." This is done on the fly and is completely transparent to the Altair computer and software. The soft sectored media can be exchanged between aligned drives as long as an FDC+ is used (the soft sectored media will not work with the original Altair two-board FDC). Belt driven 8" drives like the Shugart 80x are not reliable with soft sector media due to speed variations within a revolution. The same speed variations exist with belt driven 5.25" drives, however, the sector timing of the smaller drives is more forgiving, so the soft sectored media tends to work reliably with the 5.25" drives.

A 96 TPI, high-density 5.25" drive, like the Teac 55-GFR, can be used with readily available soft sectored media as a transparent replacement for an Altair 8" drive. The Altair computer and software cannot tell it's not connected to an original Altair 8" drive. Of course, the media is not interchangeable with an 8" drive, but the media is interchangeable between the 5.25" drives. This allows you to run all the original Altair floppy-based software, use and handle floppy disks, but skip the hassle of repair, alignment, and maintenance of the older 8" drives. The drives are also much smaller, less expensive, require smaller power supplies, and the 5.25" soft sectored media is still readily available.

A high density 5.25" drive can also be used as a 1.5Mb drive under CP/M. In addition, a DSDD 8" drive can also be used as a 1.5Mb drive under CP/M. Note: Original Altair software does not support the 1.5Mb configuration.

The FDC+ includes a built-in, high speed serial port that can connect to a PC running a serial disk server. From the perspective of the Altair computer and software, it appears that an Altair/Pertec drive is attached. Performance of the serial drive is virtually identical to the original Altair floppy drive. Both the Altair 8" drive and Minidisk drive are supported. Numerous disk images – available via links on the FDC+ web page – can be mounted on the PC server and run from your Altair computer. Disk images include a customized version of CP/M that provides 8Mb capacity on drives A & B and standard Altair drives on C & D.

Watch the FDC+ in action

Ordering Information

The FDC+ comes fully assembled and tested as shown in the picture below. The following items are also included:

  • 27C64 EPROM pre-programmed with the Altair disk boot loader (DBL), the multi-boot loader (MBL), the turnkey monitor (TURMON), an Intel hex loader, and a full featured monitor ROM (ALTMON).
  • IDC-14 to DB-25F serial cable (for FDC+ monitor access and serial disk operation)
  • IDC-50 to DB-37F adapter for Altair 8" drives (connectors not provided)
  • IDC-50 to IDC-26 adapter for Altair Minidisk (connectors not provided)
  • IDC-50 to IDC-34 adapter for 5.25" drives (connectors not provided)

FDC+ Enhanced Floppy Controller


All prices are in US dollars. Shipping charges are additional and vary based on destination. Contact me at the email address shown at the bottom of the page to order.


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(972) 429-0947
Mike Douglas


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