Developed by Seagate. Also known as MFM or RLL since these are the encoding methods used to store data. Seagate originally developed it to support their ST506 (5 MB) and ST412 (10 MB) drives. The first drives used an encoding method called MFM (Modified Frequency Modulation). Later a new encoding method was developed, RLL (Run Length Limited). RLL had the advantage that it was possible to store 50% more with it. But it required better drives. This is almost never an problem. Often called 2,7 RLL because the recording scheme involves patterns with no more than 7 successive zeros and no less than two. This standard drive system uses two cables; a 34 conductor control cable, and a 20 conductor data cable. The control cable contains a twist of the conductors going to the farthest drive, which is drive “C” on most systems. This twist consists of conductors 25 through 29. As with the floppy cable, the ST506/412 cables normally have a key to prevent reversal, and the controller end has a pin-type connector, while the drive end has a card-edge type connector. Though control signals go through a single 34 conductor cable, data flows through separate 20 conductor cables for each drive (C,D).
34 PIN IDC MALE at the Controller
20 PIN IDC MALE at the Controller
34 PIN IDC FEMALE at the Harddisk
20 PIN IDC FEMALE at the Harddisk
Control connector
| Pin | Name |
|---|---|
| 2 | Head Sel 8 (was originally used as Reduced Write Current) |
| 4 | Head Sel 4 |
| 6 | Write Gate |
| 8 | Seek Complete |
| 10 | Track 0 |
| 12 | Write Fault |
| 14 | Head Sel 1 |
| 16 | RES (reserved) |
| 18 | Head Sel 2 |
| 20 | Index |
| 22 | Ready |
| 24 | Step |
| 26 | Drive Sel 1 |
| 28 | Drive Sel 2 |
| 30 | Drive Sel 3 |
| 32 | Drive Sel 4 |
| 34 | Direction In |
- All odd pins are GND, Ground.
Data connector
| Pin | Name | Description |
|---|---|---|
| 1 | Drive Selected | |
| 2 | GND | Ground |
| 3 | RES | (reserved) |
| 4 | GND | Ground |
| 5 | RES | (reserved) |
| 6 | GND | Ground |
| 7 | RES | (reserved) |
| 8 | GND | Ground |
| 9 | RES | (reserved) |
| 10 | RES | (reserved) |
| 11 | GND | Ground |
| 12 | GND | Ground |
| 13 | Write Data+ | |
| 14 | Write Data- | |
| 15 | GND | Ground |
| 16 | GND | Ground |
| 17 | Read Data+ | |
| 18 | Read Data- | |
| 19 | GND | Ground |
| 20 | GND | Ground |