How IDE Controllers Work

Cable Key

The connector on an IDE cable
The connector on an IDE cable

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.

Pin Numbers and Descriptions

  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 (pin missing)
  21. DRQ 3
  22. Ground
  23. -IOW
  24. Ground
  25. -IOR
  26. Ground
  27. I/O Channel Ready
  28. SPSYNC: Cable Select
  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 (Logic) (Optional)
  42. +5 Volts (Motor) (Optional) 
  43. Ground (Optional)
  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.