You probably came here looking for the standard 34 pin floppy drive pinout, as used on most PCs. However, the PC floppy drive pinout was derived from a standard created by shugart in the 1970s, and More than one version of that pinout exists today. So let’s talk about the common floppy drive pinout, and the variance that you might see in your retro adventures.

The IBM PC floppy drive pinout

Floppy drive pinouts

The 8-inch Shugart mechanism on the far left dwarfs the full-height 5.25-inch mechanism from the IBM PC in the center, and a 3.5-inch floppy drive on the right. The pinouts for all these drives are all based on the original Shugart floppy drive on the left, but many manufacturers changed them slightly to suit their own purposes.

When IBM developed its first PC, they sourced floppy drives from the same people everyone else did, Shugart. But IBM hacked the standard to a slight degree to make the drives a bit easier to install. They sliced the cable and twisted four of the lines, 10-14,  to select which drive is drive A and which is drive B. This eliminated the need to change jumpers when installing a second drive.

The PC became so dominant that this became the de facto standard floppy connector pinout, even though it violated the original design. This is why modern drives are hardwired to drive 1 while old drives have jumpers to set the drive number.

IBM was neither the first nor the last to make subtle changes to the standard, usually involving the drive select pins or reappropriating ground or previously unused pins for supplying power.

The discussion is largely academic unless you are attempting to adapt newer PC hardware for other purposes.

1Ground2Density
3Ground4Reserved
5Ground6Reserved
7Ground8Index
9Ground10Drive motor 0
11Ground12Select drive 0
13Ground14Select drive 1
15Ground16Drive motor 1
17Ground18Direction
19Ground20Step
21Ground22Write Data
23Ground24Write Enable
25Ground26Track 0
27Ground28Write Protect
29Ground30Read Data
31Ground32Side Select
33Ground34Disk Change

The Tandy 1000 floppy pinout

The Tandy 1000 was famously more IBM compatible than the IBM PCjr it was designed to compete with. But the one place Tandy deviated from this rule was with their three and a half inch floppy drives. Tandy reappropriated 9 ground pins on the 34-pin connector for 3.5-inch drives for transmitting power. This cut down on the amount of cabling necessary, which was an advantage in its smaller form factor systems. But it can give us trouble today. If you plug a floppy cable backwards into a Tandy, you will damage the drive by sending power to lines that aren’t expecting it.

So if you’re not sure where pin 1 is, and pin 1 isn’t marked clearly on the drive, use your multimeter to check for continuity between the odd-numbered pins. If pins 1 and 11 are connected but they are not connected to pin 13, it’s a Tandy 1000 drive with voltage on pins 1, 3, 5, 7, 9, and 11.  And then I recommend you clearly mark pin 1 on both the cable and your drives.

1+5V2Not connected
3+5V4Not connected
5+5V6Not connected
7+5V8Index
9+5V10Select drive 0
11+5V12Select drive 1
13Ground14Not connected
15Ground16Drive motor on
17Ground18Direction
19Ground20Step
21Ground22Write Data
23Ground24Write Enable
25Ground26Track 0
27Ground28Write Protect
29+12V30Read Data
31+12V32Side Select
33+12V34Disk Change

The IBM PS/2

Of course the IBM PS/2 would be non-standard. IBM did the same thing Tandy did and to cut down on the number of wires inside the system, they reallocated some ground pins to power. This introduces an incompatibility between drives intended for the rest of the market if you want to use one in a PS/2. It also means if you manage to plug a PS/2 floppy cable in backwards, you will damage the drive. As PS/2 parts are getting rarer and more expensive, you don’t want to do that.

1Ground2Density
3+5V4Drive Type
5Ground6+12V
7Ground8Index
9Ground10Drive motor 0
11Ground12Select drive 0
13Ground14Select drive 1
15Ground16Drive motor 1
17Ground18Direction
19Ground20Step
21Ground22Write Data
23Ground24Write Enable
25Ground26Track 0
27Ground28Write Protect
29Ground30Read Data
31Ground32Side Select
33Ground34Disk Change

Atari ST

The Atari ST floppy drive is extremely similar to the PC floppy drive, and they used a nearly identical disc format. This is because the operating system was developed by digital research, a long time Microsoft competitor, and the ST operating system was itself a derivative of digital research’s products for the IBM PC.

The ST used a very different processor, so there was a fair bit of development work that went into adapting digital researches software to the Atari hardware. About Atari used a lot of off-the-shelf hardware in the ST, and making a lot of changes to the floppy drive didn’t make sense.

So here is the Atari ST floppy connector pinout.

1Ground2Disk Change
3Ground4Not Connected
5Ground6Not Connected
7Ground8Index
9Ground10Select drive 0
11Ground12Select drive 1
13Ground14Not Connected
15Ground16Drive motor
17Ground18Direction
19Ground20Step
21Ground22Write Data
23Ground24Write Enable
25Ground26Track 0
27Ground28Write Protect
29Ground30Read Data
31Ground32Side Select
33Ground34Ready

Amiga

The Amiga was its own thing, but it’s designers changed it’s floppy drive less than they changed The rest of the stuff in the box.

Standard PC floppy drives do need some modification to work in an Amiga, and you cannot use a PC high density drive as an Amiga high density drive, because of the difference in data rates. But you can use a PC identity drive as an Amiga double density drive without any problems.

It’s still worth doing because PC floppy drives cost much less than Amiga floppy drives.

1Ground2Disk Change
3Ground4Motor on
5Ground6Not Connected
7Ground8Index
9Ground10Select drive 0
11Ground12Select drive 1
13Ground14Not Connected
15Ground16Drive motor
17Ground18Direction
19Ground20Step
21Ground22Write Data
23Ground24Write Enable
25Ground26Track 0
27Ground28Write Protect
29Ground30Read Data
31Ground32Side Select
33Ground34Ready

8 inch floppies

The very first floppy drives used a disk 8 inches across. They also used a slightly different pinout. The original connector had 50 pins rather than 34. A large number of those pins were not used, so later drives only used 34. Even with 34 pins, a lot of them were extra.

1Ground2Track Greater than 43
3Ground4Not Connected
5Ground6Not Connected
7Ground8True Ready
9Ground10Two Sided
11Ground12Disk Change
13Ground14Side Select
15Ground16In Use
17Ground18Head Load
19Ground20Index
21Ground22Ready
23Ground24Sector
25Ground26Drive Select 1
27Ground28Drive Select 2
29Ground30Drive Select 3
31Ground32Drive Select 4
33Ground34Direction
35Ground36Step
37Ground38Write Data
39Ground40Write Enable (or Gate)
41Ground42Track 0
43Ground44Write Protect
45Ground46Read Data
47Ground48Sep Data
49Ground50Sep Clock

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