The 34-pin floppy disk interface was developed by Shugart Associates for the SA400 5.25-inch minifloppy drive and became the de facto standard for PC floppy drives. The connector is a 34-pin PCB edge card or IDC header; all odd-numbered pins are ground returns. Signalling is TTL-compatible and active-low with open-collector outputs: logic true is 0.0–0.4 V and logic false is 2.5–5.25 V, with 150-ohm input impedance. The original design requires each drive to be hardware-jumpered to a unique Drive Select address; IBM introduced a 7-wire cable twist (conductors 10–16 reversed at the Drive 1 connector) so that both drives could be jumpered identically as Drive 2, with the twisted connector presenting the end drive as Drive 1.
The floppy ribbon runs from the controller to the middle (Drive 2) connector and then, through a seven-conductor twist, to the end (Drive 1) connector. Because both drives are jumpered identically (DS1), the twist is what makes the end drive appear as A: and the middle drive as B:. The twist reverses conductors 10–16:
- 10 ↔ 16 (Motor Enable A / Motor Enable B)
- 11 ↔ 15
- 12 ↔ 14 (Drive Select A / Drive Select B)
- 13 stays (Ground)
34 PIN IDC FEMALE to the Controller
34 PIN IDC FEMALE to the Drive 2
34 PIN IDC FEMALE to the Drive 1
| Controller | Drive 1 | Drive 2 | |
|---|---|---|---|
| Wire 1-9 | 1-9 | 1-9 | 1-9 |
| Wire 10 | 10 | 16 | 10 |
| Wire 11 | 11 | 15 | 11 |
| Wire 12 | 12 | 14 | 12 |
| Wire 13 | 13 | 13 | 13 |
| Wire 14 | 14 | 12 | 14 |
| Wire 15 | 15 | 11 | 15 |
| Wire 16 | 16 | 10 | 16 |
| Wire 17-34 | 17-34 | 17-34 | 17-34 |
Notes
- The connector at the controller end and the Drive 2 connector are straight-through; the Drive 1 connector incorporates the 7-wire twist on conductors 10–16.
- If only one drive is used, leave the middle (Drive 2) connector unoccupied and attach the drive to the twisted end.
- The last drive in the daisy-chain must carry 150-ohm termination resistors; remove them from all other drives.
- Cable may be flat ribbon (e.g. 3M Scotchflex) or twisted pair; maximum recommended length is 10 feet (approximately 3 metres).
- The physical connector is a 34-pin IDC header on most PC drives or a PCB edge card connector on older 5.25-inch drives.
Signals
Even-numbered pins carry the active signals; all odd-numbered pins are ground returns.
- Drive Select 1–4 (DS1–DS4) — multiplexed lines that select which drive is active; in the IBM cable-twist scheme, all drives are jumpered as DS2 and the twist rewires the end drive to respond as DS1
- Motor On — commands the drive spindle motor to run
- Direction Select — sets the direction of head stepping (inward towards higher tracks, or outward)
- Step — each active pulse moves the read/write head one track in the selected direction
- Write Data — serial data stream to be written to the diskette surface
- Write Gate — enables write circuitry when asserted; must be deasserted during stepping
- Track 00 — output asserted by the drive when the head is positioned over track 0
- Write Protect — output asserted by the drive when a write-protected diskette is inserted
- Read Data — serial data stream read from the diskette surface
- Index/Sector — pulse generated once per spindle revolution indicating the start of the track