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Saturday, September 13, 2008

Enhanced Small Device Interface

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

ESDI 34-pin Control Connector Pinout
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
ESDI 20-pin Data Connector Pinout
~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