View Full Version : FDADAP in reverse

November 2nd, 2015, 03:17 PM
Quick question (and probably a stupid one),
Just wondering if a FDADAP can be used in reverse? Or in other words...Can a 3.5" (or 5.25") drive be connected through a FDADAP to a S-100 8" controller board (in my case either a Versafloppy 1 (SSSD) or Morrow DJ2D (DSDD) board).

Just thinking to try an experiment to replace the 8" drives with a 3.5", and wondering if my FDADAP would be useful.


November 2nd, 2015, 05:46 PM
Hi All;

Nama, I don't know or have any idea what a FDADAP is..
Not even a clue..


November 2nd, 2015, 05:49 PM
No, but you don't need it for that anyway! There are a number of 3.5" floppies that can be jumpered for direct compatibility with 8" drive controllers. Herb Johnson has information about the conversion on his site:


November 2nd, 2015, 05:57 PM
AFAIK, not useful. Simple enough to construct a new cable that will work--the 3.5" signals are much more straightforward.

November 2nd, 2015, 06:03 PM
Hi Nama,

I am not convinced that hooking the FDADAP up in reverse would be successful. The onboard microcontroller generates the TG43 signal needed by a downstream 8" floppy as well as combining the 'motor on' signals. You would also need an extra 3.5" floppy power connector to power the FDADAP that the S100 system wouldn't supply. Hooking all of this up in reverse may not only damage the FDADAP but your S100 floppy controller too.

Go to the FDADAP website and use the info@dbit.com link to contact the maker to see if your idea will work before trying it.

Hope this helps,


November 2nd, 2015, 06:21 PM
Hi All;

Nama, I don't know or have any idea what a FDADAP is..
Not even a clue..


Google is your friend

November 2nd, 2015, 06:26 PM
Thanks guys,
I'd found Herb's site, and I am presently working through the various threads he's archived on the subject.

I suspected that hooking the FDADAP in reverse would probably damage something, but thought I'd pose the question here incase anyone has had experience with such things. Certainly wouldn't want to damage any hardware.

Thanks again


November 2nd, 2015, 07:28 PM
Unfortunately, in life some processes aren't easily reversible. You can't run your juicer in reverse and get real oranges from orange juice.

But take heart--making a cable adapter is pretty simple, particularly if the connection to your disk controller is a simple header. Basically, it's get a length of ribbon cable, two female IDCs (50 and 34 position); crimp one and then practice your weaving. Signal names are the same; use the 8" head load signal for the 3.5" motor control. Your setup may use the 8" DS0 and DS1 (first two drive selects) and your 3.5" drive may accept only DS1, so it'd be 8" DS0->3.5" DS1. Most HD 3.5" drives don't have a separate ready signal, so you'll have to take the 8" READY input and tie it to ground, so that the drive always appears to be ready.

November 3rd, 2015, 07:21 AM
This thread right here in the Tandy forum on this very site: http://www.vintage-computer.com/vcforum/showthread.php?48252-Model-II-12-16-16B-6000-8-quot-to-5-25-Inch-floppy-adapter-cable-50-pin-to-34-pin might help. Towards the end, there are links for a simple board that allows you to use off the shelf cables, a 34-pin on one side and a 50-pin on the other. The Model II used standard Shugart 8 inch drives at the first.

November 3rd, 2015, 10:04 PM
I was just sitting down to map out how I was going to build a cable from Chuck(G)'s and Herb's instructions, when I saw your post. Brilliant. Will order some words for sure.