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IDE (also known as Integrated Drive Electronics) was created through a collaboration of Compaq and Western Digital.   IDE has become the standard for the use of hard drives in computers.

                     IDE controller (on the hard drive unit)

                     Image by CompuClues staff

Most Motherboards today have a IDE interface on them. The interface on the motherboard is connected to the controller on the hard drive with a 40 pin cable.

  From: KJRyan

There are many different kinds of IDE; some are listed below.

• ATA-1: It used the use of master and slave configuration. ATA-1 was based on a subset of the standard ISA 96-pin version using 40 or 44 pin connectors. In the 44-pin version, the other four pins are used as a power supply to the drive. Also, ATA-1 provides signal timing for DMA and programmed input/output functions.
• ATA-2: DMA was fully used with the beginning of the ATA-2 version. The standard DMA transfer rate was increased from 4.16 megabytes per second to 16.67 megabytes per second. ATA-2 has power management, PCMCIA card and removable device support. ATA-2 is also called EIDE, Fast ATA or Fast ATA-2. The supported drive size also increased to a full 137.4 gigabytes. ATA-2 also provided Cylinder Head Sector for drives up to 8.4 gigabytes.
• ATA-3: ATA-3 also introduces the use of SMART Technology, making IDE drives more reliable.
• ATA-4: ATA-4 introduced Ultra DMA and the AT Attachment Program Interface (ATAPI). ATAPI provides an interface for CD-ROM drives, tape backup, and removable drives. Ultra DMA increased the transfer rate to 33.33 megabytes per second too. The cable also has 80 pins. The other forty are used for ground wires.
• ATA-5: The major thing in ATA-5 is auto detection of which cable is used. Ultra DMA is also increased to 66.67 megabytes per second.
  

From:  CompuClues Staff

IDE devices use ribbon cable to connect the interface to the controller on the hard drive unit. The 40 pin connector is standardized.

IDE ribbon cables come in two flavors (40 lead and 80 lead.)

(80 Leads) (40 Leads) 
Image by CompuClues staff

  From: KJRyan

PIN: DESCRIPTION:
1 Reset
2 Ground
3 Data Bit 7
4 Data Bit 8
5 Data bit 6
6 Data Bit 9
7 Data bit 5
8 Data Bit 10
9 Data Bit 4
10 Data Bit 11
11 Data Bit 3
12 Data Bit 12
13 Data Bit 2
14 Data Bit 13
15 Data Bit 1
16 Data Bit 14
17 Data Bit 0
18 Data Bit 15
19 Ground
20 Cable Key
21 DRQ 3
22 Ground
23 -IOW
24 Ground
25 -IOR
26 Ground
27 I/O Chanel Ready
28 SPSYNC: Cable Seclect
29 -DACK 3
30 Ground
31 RQ 14
32 -IOCS 16
33 Address Bit 1
34 -PDIAG
35 Address Bit 0
36 Address Bit 2
37 -CS1FX
38 -CS3FX
39 -DA/SP
40 Ground
41 +5 Volts (Optional)
42 +5 Volts (Optional)
43 Ground (Optional)
44 -Type (Optional)