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 | +5V | 2 | Not connected |
| 3 | +5V | 4 | Not connected |
| 5 | +5V | 6 | Not connected |
| 7 | +5V | 8 | Index |
| 9 | +5V | 10 | Select drive 0 |
| 11 | +5V | 12 | Select drive 1 |
| 13 | Ground | 14 | Not connected |
| 15 | Ground | 16 | Drive motor on |
| 17 | Ground | 18 | Direction |
| 19 | Ground | 20 | Step |
| 21 | Ground | 22 | Write Data |
| 23 | Ground | 24 | Write Enable |
| 25 | Ground | 26 | Track 0 |
| 27 | Ground | 28 | Write Protect |
| 29 | +12V | 30 | Read Data |
| 31 | +12V | 32 | Side Select |
| 33 | +12V | 34 | Disk Change |
Attribution:
The Silicon Underground
David L. Farquhar