If you go to change or add drives in a vintage PC and your floppy cable doesn’t fit, there could be several reasons for it. Here’s what to do about it when this happens.
PC floppy cables are all 34 pins, but they can have several different ways of keying so you don’t plug them in wrong. This can cause physical compatibility issues that may require modifying the cable so it will plug into your motherboard.
Three ways of keying
Plugging a floppy cable in backwards was harmless (with a couple of exceptions–more on that in a minute), but the drives wouldn’t work. It just sent all the drive signals to ground. Unfortunately, if you offset the cable, sometimes it can damage the drive permanently because it could send signals to pins that couldn’t handle them. Keying the cable so it only fit one way reduced support calls and warranty claims.
There were three ways of keying.
One was to put a tab on the cable itself, which mated with a slot on the floppy connector shroud on the motherboard. IDE hard drive cables often used the same kind of arrangement. This was effective, but increased the cost of motherboards slightly.
Another method was to leave out a pin on the motherboard and on the floppy cable. This made it physically impossible to plug the cable in wrong. Officially, it was pin 3 that was supposed to be used for this purpose. I’ve also seen pin 5, officially a ground pin, used for this purpose. I think I may have also seen pin 17 used this way, similarly to a hard drive cable, but I may be misremembering.
The Tandy and PS/2 caveat
Plugging a floppy drive cable backwards on standard PCs was harmless. But on a Tandy 1000 or IBM PS/2 it wasn’t. Tandy and IBM repurposed some unused pins for power, to eliminate the separate power cable. This made cable management a lot easier, but if you plug one of those cables in backwards, you’re going to damage a drive.
Modifying a cable to fit
You could modify your motherboard or drive to take the cable, but it’s better to modify a $2 cable than a vintage motherboard or drive. The motherboard is worth more. The method to use depends on what’s causing the incompatibility.
If the cable has a tab and your motherboard’s connector shroud doesn’t have a slot to match it, just grind off the plastic tab with a file or rotary tool, or carefully whittle it off with a sharp knife. As long as you don’t dig into the plastic, this won’t affect compatibility at all.
If the cable has a pin filled in that your motherboard expects to be open, just drill out that blocked position in the cable. Use a 1/16 or smaller drill bit. You won’t have to drill far.
After you make this modification, the cable should plug in just fine. Just be cognizant of which pin is pin 1 and line it up correctly. Pin 1 on the cable has the red stripe. At the very least, make sure you don’t offset the cable. Most of these drives aren’t being made anymore so you don’t want to damage them. I find it helpful to use a cheap Harbor Freight worklight. Having lots of light makes it easier to make sure you connected it properly.
Modifying cables is something you have to do a fair bit in retro computing. Sometimes you have to modify monitor cables too.