IDE=Integrated Drive Electronics.
Developed by Compaq and Western Digital.
Newer version of IDE goes under the name ATA=AT bus Attachment.
Often, the control circuitry is built into the motherboard, eliminating the requirement for a separate Host Adapter.
There are 2 types of IDE interfaces...those for the 8-bit XT bus, and those for the 16-bit AT bus (detailed here).
The cable for IDE contains 40 conductors and has no twists.
Like an SCSI cable, the IDE cable uses a Dual-row Pin connector for both ends.
A single cable may be used to connect two drives, or two cables may be Daisy-Chained.
Most IDE Host Adapters will support two hard drives.
The first drive should be jumpered as the Master drive, and the second as the Slave drive.
Plug-in IDE Host Adapters are often called Paddle-Boards, and may contain a floppy controller, and serial and parallel ports. 
(40 PIN IDC MALE at the controller & peripherals)
(40 PIN IDC FEMALE at the cable)
| Pin | Name | Dir | Description | | 1 | /RESET | OUT | Reset | | 2 | GND | ------ | Ground | | 3 | DD7 | IN/OUT | Data 7 | | 4 | DD8 | IN/OUT | Data 8 | | 5 | DD6 | IN/OUT | Data 6 | | 6 | DD9 | IN/OUT | Data 9 | | 7 | DD5 | IN/OUT | Data 5 | | 8 | DD10 | IN/OUT | Data 10 | | 9 | DD4 | IN/OUT | Data 4 | | 10 | DD11 | IN/OUT | Data 11 | | 11 | DD3 | IN/OUT | Data 3 | | 12 | DD12 | IN/OUT | Data 12 | | 13 | DD2 | IN/OUT | Data 2 | | 14 | DD13 | IN/OUT | Data 13 | | 15 | DD1 | IN/OUT | Data 1 | | 16 | DD14 | IN/OUT | Data 14 | | 17 | DD0 | IN/OUT | Data 0 | | 18 | DD15 | IN/OUT | Data 15 | | 19 | GND | ------ | Ground | | 20 | KEY | - | Key | | 21 | n/c | - | Not connected | | 22 | GND | ------ | Ground | | 23 | /IOW | OUT | Write Strobe | | 24 | GND | ------ | Ground | | 25 | /IOR | OUT | Read Strobe | | 26 | GND | ------ | Ground | | 27 | IO_CH_RDY | IN | | | 28 | ALE | OUT | Address Latch Enable | | 29 | n/c | - | Not connected | | 30 | GND | ------ | Ground | | 31 | IRQR | IN | Interrupt Request | | 32 | /IOCS16 | ? | IO ChipSelect 16 | | 33 | DA1 | OUT | Address 1 | | 34 | n/c | - | Not connected | | 35 | DA0 | OUT | Address 0 | | 36 | DA2 | OUT | Address 2 | | 37 | /IDE_CS0 | OUT | (1F0-1F7) | | 38 | /IDE_CS1 | OUT | (3F6-3F7) | | 39 | /ACTIVE | OUT | Led driver | | 40 | GND | ------ | Ground |
Note: Direction is Controller relative Devices (Harddisks).
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