Cable Key
IDE devices use a ribbon cable to connect to each other. Ribbon cables have all of the wires laid flat next to each other instead of bunched or wrapped together in a bundle. IDE ribbon cables have either 40 or 80 wires. There is a connector at each end of the cable and another one about two-thirds of the distance from the motherboard connector. This cable cannot exceed 18 inches (46 cm) in total length (12 inches from first to second connector, and 6 inches from second to third) to maintain signal integrity. The three connectors are typically different colors and attach to specific items:
  • The blue connector attaches to the motherboard.
  • The black connector attaches to the primary (master) drive.
  • The grey connector attaches to the secondary (slave) drive.
Along one side of the cable is a stripe. This stripe tells you that the wire on that side is attached to Pin 1 of each connector. Wire 20 is not connected to anything. In fact, there is no pin at that position. This position is used to ensure that the cable is attached to the drive in the correct position. Another way that manufacturers make sure the cable is not reversed is by using a cable key. The cable key is a small, plastic square on top of the connector on the ribbon cable that fits into a notch on the connector of the device. This allows the cable to attach in only one position.

The connector on an IDE cable

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

Note that the last four pins are only used by devices that require power through the ribbon cable. Typically, such devices are hard drives that are too small (for example, 2.5 inches) to need a separate power supply.