The first attempt at improving the original ST-506/ST-412 hard disk interface was the Enhanced Small Device Interface or ESDI. ESDI was developed in the mid-1980s by a consortium of hard disk manufacturers led by Maxtor. It was eventually codified as an ANSI standard; the peak of its popularity was in the late 1980s.
ESDI improved on ST-506/ST-412 in several ways. It moved some drive controller functions to the hard disk from the controller card, eliminating some of the reliability problems associated with its predecessor. It had a maximum theoretical bandwidth of 24 Mbits/second (fairly fast for those days), though in practice the limit was about half of that. There were other added features and small performance enhancements as well. Its primary design still had almost all of the intelligence on the controller and not on the hard disk.
While ESDI was a real improvement over the older ST-506/ST-412 interface, it was "too little, too late" in a lot of ways. In the late 1980s ESDI suffered under competition from IDE/ATA in the mainstream market and from SCSI in the high-end market, both of which offered significant advantages over ESDI, such as simpler configuration, lower cost and improved performance. As a result, by the early 1990s ESDI had been all but wiped off the interface map.
Connector Pinouts
| GROUND | 1 | 2 | ~HD SLCT 3 |
| GROUND | 3 | 4 | ~HD SLCT 2 |
| GROUND | 5 | 6 | ~WRITE GATE |
| GROUND | 7 | 8 | ~CNFG/STATUS |
| GROUND | 9 | 10 | ~XFER ACK |
| GROUND | 11 | 12 | ~ATTENTION |
| GROUND | 13 | 14 | ~HD SLCT 1 |
| Key (no pin) | 15 | 16 | ~SECTOR |
| GROUND | 17 | 18 | ~HD SLCT 1 |
| GROUND | 19 | 20 | ~INDEX |
| GROUND | 21 | 22 | ~READY |
| GROUND | 23 | 24 | ~XFER REQ |
| GROUND | 25 | 26 | ~DRV SLCT 0 |
| GROUND | 27 | 28 | ~DRV SLCT 1 |
| GROUND | 29 | 30 | Reserved |
| GROUND | 31 | 32 | ~READ GATE |
| GROUND | 33 | 34 | ~CMD DATA |
| ~DRV SLCTD | 1 | 2 | ~SECTOR |
| ~CMD COMPL | 3 | 4 | ~ADDR MK EN |
| GROUND | 5 | 6 | GROUND |
| +WRITE CLK | 7 | 8 | -WRITE CLK |
| GROUND | 9 | 10 | +RD/REF CLK |
| -RD/REF CLK | 11 | 12 | GROUND |
| +NRZ WRITE | 13 | 14 | -NRZ WRITE |
| GROUND | 15 | 16 | GROUND |
| +NRZ READ | 17 | 18 | -NRZ READ |
| GROUND | 19 | 20 | ~INDEX |
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