Unless you are limiting your vintage computer hobby to systems manufactured after the early to mid 1980s at some point you will come across an 8" floppy disks and the associated drives. These were one the earliest models of FDDs and came in many flavors: FM, MFM, SS, DS, hard sectored, etc. My interest is mainly in IBM produced machines after the mid 70s. During that time IBM was using DSDD 8" drives in a number of their systems (e.g. the System 360, the 5110/5120, the 5322/5324 and their associated external drives). As you can imagine getting SW for these systems is almost impossible, so if you want a usable system you need a way to preserve and duplicate 8" disks.

Unfortunately unlike 5 1/4" and 3 1/2" drives whose standard sizing, electrical connections, and mounting points have survived to this day there is no easy way to install an 8" drive in a modern system. As such you will either need a custom made drive cage and computer case or drives mounted in an external enclosure. Some people will of course forgo all that fanciness and just place the bare drive out on a table.

To complicate things further 8" drives use a 50pin header (usually card edge) and require special signaling to write a disk properly. A modern HD floppy controller, while capable of reading an 8" floppy, cannot properly write one. This where the D Bit FDADAP comes into play; to quote the D Bit site:

The D Bit FDADAP board is a small adapter which adapts 8" floppy disk drives (Shugart SA800 style bus) to work with the PC 3.5"/5.25" floppy disk cable pinout... Some 8" floppy disk drives require a "TG43" signal, which tells the drive whether its head carriage is currently positioned at a track number greater than 43... This signal is not normally provided by PC FDCs, so the FDADAP board has a microcontroller which monitors the disk bus and keeps track of the signals related to seeks, so that it knows which track each drive is positioned on, and transparently generates the correct TG43 signal for the currently selected drive with no intervention from host software.

In essence FDADAP is an adapter to allow the use of 8” drives with a more modern FDC. Now that we know what the FDADAP does let us take a quick look at the card –


And for the curious some measurements:

1.8" Wide3.5" Tall
3.26" From Hole to Hole

I purchased my FDADAP so that I could connect a pair of Qume 842s, in an external enclosure, to an IBM PC XT. Please note that the standard XT FDC does not support HD drives and as such cannot work with 8” drives even with a FDADAP. I will be using a CompatiCard II to this end. My external enclosure provides both the AC and 24V DC voltage needed to run the drives and converts the 50-pin edge connector to a 50-pin IDC. The simplest thing would have been to pull a 50-pin flat cable out of one of the expansion slots and connect to the external IDC on the enclosure.

Unfortunately I don’t do simple.

Plus I wanted something a bit more solid and permanent and I’ve never been a fan of the flat cable outside of the case. After looking around for a while I found a number of adapters and converters to get things done the way I wanted. The first thing I did was to purchase an internal 50-pin IDC to external HD50 adapter off of eBay. These are relatively common and cheap:

IDC 50 to HD50 FrontIDC 50 to HD50 Back
HD50 Connector3.19" Wide

Next I needed a way to connect the FDADAP to the HD50 adapter. I crimped a small length of 50-pin flat cable to achieve this:

Cables and AdaptersAdapters Connected with Wire

While this works it proved a bit flimsy. Luckily I have access to some sheet metal cutting tools and was able to make a back plate for the two adapters. I won’t bore you with all the details and the painting suffice to say in the end it looked like this:

Metal Back Plate

Even though the metal was primered and painted I was still a bit worried about a short so I insulated the back of the two adapters before mounting them on the plate with screws:

Insualted Back SideInstalled on Back Plate

A bit of insulation on the back as well just to be safe:

Back Plate Insulated

Next I needed a cable to connect the HD50 to the 50-pin IDC on the enclosure. Unfortunately such a cable does not exist (at least I could not find one). However, HD50 to Centronics 50 cables are abundant and cheap and luckily so are crimp-able Centronics 50 connectors:

HD50 ConnectorCentronics Connector

And the final assembled product:

Final Assembled

This provided an easy to install/remove adapter card that will fit any standard PC case while at the same time providing an external connector for easy connection of drives.