Technology and retro computer blog

FDC+ Enhanced Floppy Controller for the Altair 8800

The FDC+ is a new floppy disk controller board for original Altair 8800 computers.  It was developed by Mike Douglas, see the FDC+ website and Altair Clone website for more details.  The original Altair 8800 floppy disk controller was a two board s100 configuration which only connected to the original MITS 88-DCDD drive.  In contrast the FDC+ allows original Altair 8800  computers to connect to a variety of floppy disk drives from the original MITS 8" floppy drive to more recent TEAC FD-55GFR 5.25" drives.  It also allows the streaming of disk images to the Altair via a serial link, so you can use disk images without a physical disk drive at all.


My FDC+ arrived yesterday and I spent some time today getting it up and running.  My goal  was to use a Teac 5.25" drive to act as an Altair 8" drive and to create a CP/M boot disk.  The FDC+ comes with several adapter pcb boards for use with different drives.  You select the required drive configuration and then solder the appropriate connectors onto the bare pcb.  The pcb for use with TEAC 5.25" drives requires two IDC sockets, the first is a 50 pin socket for connection to the FDC+ board and the second is a 34 pin socket for connection to the drive.  I created the required adapter for the TEAC drive.


To get things up and running quickly and easily I decided to configure the Altair with just the CPU, FDC+ and SSMIO4 serial card.  I enabled both ram and prom on the FDC+ and configured it for the correct drive type.  I have a working Teac FD-55GFR(149) so I set the jumpers as per the FDC+ manual.  I used a really short 50 pin scsi cable to connect the FDC+ to the adapter board, it actually works well because it just suspends the adapter board next to the FDC+.  I then used a standard pc 34 pin floppy cable from the adapter board to the drive.

To get CP/M onto disk I used the following approach, I booted the Altair and ran the hex loader program in the FDC+ prom.  Using the hex loader I then loaded PC2Flop.  I then ran PC2Flop and created a CP/M boot disk.  One thing I noticed is that I'd configured my drive to be drive 1 not 0.  Pc2Flop worked fine once I selected drive 1.  After creating the boot disk, I examined the combined disk boot loader and tried to boot, but realizing that my drive was configured to be drive 1 not drive 0 I had to correct the jumpers on the drive.   Once I did that I was able to boot CP/M.


I used DS/DD 5.25" disks in the TEAC drive and while it initially worked, I found on subsequent attempts to create various disks, the process would quite often fail.  After some discussions Mike suggested removing the LG jumper from the TEAC drive.  Once I did this it made the disk creation process quite reliable.  Although in further discussions it was suggested that ideally high density 5.25" disks should be used instead of double density disks.  I have yet to try the HD media to compare results.  But for now DD disks seem to be working fine.



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